Jem: Don't call me the next Dido

It's 11am on Saturday and the singer-songwriter Jemma Griffiths, aka Jem, has been up for hours. Later in the day, she will make her all-important debut on British television as she performs her new single for Channel 4's Sunday-afternoon youth strand T4. The recording will last no more than three minutes, but, as is the way in television, hours of tedious preparation are required. Every nanosecond of Jem's day is accounted for, from hair and make-up (11am-1pm) and the drive across London (1-1.30pm) to lunch and sound-check (2pm) followed by the performance itself (4pm). The schedule is implemented with military precision by her manager and older sister, Chloe. "I call her Simon Cowell," Jem giggles. "She's very honest, sometimes to the point of harshness. But I respect her opinions and I trust her, which I think is important."

It's 11am on Saturday and the singer-songwriter Jemma Griffiths, aka Jem, has been up for hours. Later in the day, she will make her all-important debut on British television as she performs her new single for Channel 4's Sunday-afternoon youth strand T4. The recording will last no more than three minutes, but, as is the way in television, hours of tedious preparation are required. Every nanosecond of Jem's day is accounted for, from hair and make-up (11am-1pm) and the drive across London (1-1.30pm) to lunch and sound-check (2pm) followed by the performance itself (4pm). The schedule is implemented with military precision by her manager and older sister, Chloe. "I call her Simon Cowell," Jem giggles. "She's very honest, sometimes to the point of harshness. But I respect her opinions and I trust her, which I think is important."

You may not have heard of Jem yet, though the chances are that within a few months there'll be no escaping her. In a Dido-esque reversal of convention, this Cardiff-born singer is already a huge star in the United States, where she has sold nearly 250,000 copies of her debut album Finally Woken. There her fans include Ellen DeGeneres, on whose TV show she has appeared, and the influential LA DJ Jason Bentley who helped break Damien Rice and Coldplay.

With a combination of raw talent and a hefty promotional campaign, Jem and her record company hope to duplicate the success back home and turn her into a household name. Her face is already beginning to adorn the nation's billboards while her new single "They", a catchy blend of soul, pop and hip-hop, is on heavy rotation on Radio 1. When it comes to television, T4 is just the beginning. She's due to perform her single on Top of the Pops tonight and record an interview for MTV. One imagines it's only a matter of time before she gets the Parkinson stamp of approval.

There is possibly no one more excited at Jem's progression from law student to platinum-selling pop star than Jem herself. I'm booked in to talk to her during the hair and make-up session, though it soon becomes clear that two hours isn't nearly enough to get the full life-story. Ask her the most mundane question and she'll take a good 10 minutes to answer it.

She doesn't so much wander off the point as embark on a long hike in the opposite direction. What is clear amid the stream-of-consciousness rambling, however, is that Jem is having the time of her life. In the past 18 months she's shaken hands with Kiefer Sutherland (she's addicted to the show 24), met the brother of her all-time hero, the late comedian Bill Hicks, and contributed to the soundtrack of Desperate Housewives. She has also appeared as a wedding singer on the teen drama du jour The OC. When she was asked to do it she "nearly died laughing" but then decided, what the hell? After she had finished filming, she was amazed to find the actress Rachel Bilson, who plays Summer, queuing to get her autograph.

At 29, Jem is, compared with her peers, slightly long in the tooth to be starting a pop career. The record industry is notoriously phobic when it comes to women over the age of 25. For Jem, however, age has never posed a problem.

"I don't think anyone believed me when I said I was in my late twenties," she recalls. "I've always looked like a child. But I think my age is an advantage. Up until now I've had a normal life. It's not like I've been doing this since I was three. I've done a degree, I've done crap jobs and I've slept on friend's floors for two years. I've come to it all with my brain intact. This way around you don't have people moulding you, you make the music you want to make and you know exactly who you are. At 29 you're a lot harder to push around than when you're 19."

Jem's looks and personality are also a million miles from the stereotypical pop diva. I'd expected a glossy Joss Stone-type with wafty tresses and a 1000-watt smile, In fact, Jem is a pale, slightly goofy individual with a whimsical sense of humour and a fantastically dirty laugh.

The novelty of having a stylist and make-up artist on daily stand-by evidently hasn't worn off for Jem and, although photo-shoots don't agree with her (until last year she had never used make-up), she's learned to endure it with good humour. "Recently I've been trying to build up this Zen-like attitude where I make out like I genuinely don't care. I really think it's working. So what if I'm pulling a funny face and looking like a minger? I'm waiting for one of those photos in Heat where they catch you with sweaty armpits. Now that would be hideous."

Though Jem is quick to laugh off the trappings of celebrity, it's clear that she's fiercely ambitious. In her early twenties she set herself the goal of getting a record deal by the time she was 25 - "I knew Madonna and Sting had done it so why couldn't I? I was a few years late but I got there in the end." Three years ago, while searching for a deal, she wrote a letter to Stevie Wonder in braille asking him to listen to her demo (alas, he never replied). She also sent an e-mail to the the DJ Bentley which read "If you don't listen to my album I'm going to come in and handcuff you to your desk". Intrigued by her cheek, he got back to her within the hour. "I'm one of these people who will never give up when she wants something. I'm a bit like the Terminator. I won't go away. That's the reason I got my record deal. There was no question that it was going to happen for me. It was more a matter of 'when' than 'if'."

The youngest of three sisters, Jem was born and brought up in Cardiff. She vividly remembers the point at which she decided she would become a singer. She was sitting on a park bench in Cardiff, aged 11, when she had a run-in with some local boys. "They were on their bikes and they said something like 'Drop your drawers and ten bob's yours." It was quite funny really but, being 11, I wasn't able to think up something witty to shout back. I remember just sitting there thinking 'I'll show you.' How cheesy is that? But that feeling of wanting to be strong and do something important with my life really stayed with me."

When she was 18, Jem moved to Brighton to study law. She did it largely to please her father, who is a lawyer, and had no intention of turning it into a career. After university, she stayed in Sussex and started up the dance label Marine Parade with her friend the DJ Adam Freedland. By 1999 she had decided it was time to get her singing career started. "This is going to sound really stupid but I had this strange moment where I suddenly remembered back to the time on the park bench. It was as if I'd forgotten what I was going to do with my life. So I dropped everything and moved back to Wales to write some songs. I didn't tell anyone what I was doing, not even my family. They all thought I'd gone completely mad."

She joined the New Deal For Musicians programme, earning £50 a week, and made some demos at a community recording studio outside Cardiff. Eighteen months later she moved to London, sleeping on friends' floors and sofas, and went in search of a record deal. The next two years, she says, were "frustrating in the extreme. I had all these A&R people getting me in for meetings and building up my hopes and then doing nothing. For some reason I thought I could get a record deal in six months, which obviously wasn't the case. It turned out to be a good thing, though, because I thought I was ready and I wasn't." The next year brought a series of false starts. Annie Nightingale made the song "Finally Woken" the single of the week, though the record companies remained unmoved. Jem recorded a track with Groove Armada, which was scrapped at the last minute. Frustrated by the lack of interest, she moved to New York where she rented a room from a family in Brooklyn.

"The father really loved music so we really bonded over that. Three days later it was September 11 and he was killed in the south tower. I was with these lovely people that I hardly knew and suddenly I had become a nanny to two children who had lost their dad. I stayed for another couple of weeks and then came home. I was quite traumatised for a while."

The real turning point came when she worked with the producer Guy Sigsworth, who had just written and co-produced Madonna's single "What It Feels Like for a Girl". "What's cool about him is that he isn't all about the money," she says. "He could have worked with anyone he wanted. We couldn't pay him but he said he liked my music and decided to help me out." Indeed, Sigsworth was so impressed with "Nothing Fails" that he played it to Madonna, who promptly recorded it for her American Life album. It raised her stock considerably. Soon afterward, she decamped to Los Angeles where she finally signed a deal with Dave Matthews' ATO label.

"I have never been so happy. Chloe and I went in to play the A&R guy the album. During the first few songs song he was rubbing his face and not looking at us and then he started rocking out in his chair. In the middle of the last song he got out his wallet and handed me his gold American Express card. He said later that he was panicking that I would leave the room without signing the deal."

Jem now calls Los Angeles home though work dictates that she make frequent visits to London. She's delighted that Britain is finally waking up to her talent. "I got a text last week from a friend telling me that my face was all over the Tube," she says. "It's been going well in America for a while, though my family have been quite removed from it. But now it's happening over here they can really feel it, especially now that I'm doing Top of the Pops." She's rather less impressed that critics are hailing her the next Dido. "Don't get me wrong. She's amazing and lovely and if all of her fans want to buy my album then that's really great. I used to think 'Yeah I get it, we've both got soft voices'. But, you know, I actually watched her video last night for the first time and I was, like, 'What? I'm nothing like her'."

At this point, Chloe calls time on the interview as the car has arrived to take us to Channel 4. Three hours later, after more buffing from the hair and make-up people and lots of loitering, Jem finally has her three minutes in front of the camera. Then, as quickly as it started, it's all over. "Can you believe it?" she cries, back in her dressing room. "All that bloody fuss for just for a few seconds on the telly. Not that I'm bitter or anything." Then she gives me a nudge. "Nah, I don't mind really. If it means that more people will hear my music then it's all good, isn't it?"

'Finally Woken' is released on 21 March on ATO. The single 'They' is out on 14 March