LIKE many 21-year-olds, Chibundu Onuzo is basking in the satisfaction of having recently completed her university finals. Although she is yet to receive her results, she is turning over ideas about her future, and some of the big decisions are still up in the air. "It's something I have to pray about, to ask, 'What does God want for my life?'" she says.

She has a head start on her fellow students, however: she has already published one novel, and is working on her second. Since The Spider King's Daughter came out, a couple of months ago, her time has been divided between the demands of studying for her history degree at King's College, London, and promoting her book. And, while her tutors have been kind, "They haven't cut me any slack," she says. "There have been no extensions on my essay deadlines."

When they offered her a two-book deal, at 18 years of age, Onuzo became the youngest woman ever to be signed up by Faber & Faber. She appears remarkably unfazed by this, and points out that her fellow Nigerian author Helen Oyeyemi was published at much the same age.

Onuzo began writing as a child. She completed her first novel, set in the United States, at the age of ten. "My mum said: 'Why don't you write about what you know?' But at 11, 12, 13, Nigeria didn't seem very interesting."

But moving to the UK as a teenager to attend boarding school (St Swithun's, in Winchester, something of a family tradition) changed all that. "I wasn't at home; so home became interesting," she says. "I started setting my fiction in Nigeria."

There was an early draft of The Spider King's Daughter when she was 15 or 16 years old, but the serious work began while she was studying for A levels. "I wrote a paragraph, and I had a title, and then I left it on my laptop for a year," she says. After her AS levels, "I went back to it, and wrote from there."

SHE picked an agent out of the Writers' and Artists' Yearbook, and sent the first 33 pages. "I thought, if this agent doesn't like it, I'll stop writing it. But Capel & Land wrote back and said: 'Can we have the rest of the novel?' - this novel, which didn't exist! That encouraged me to finish it."

Completing the book meant writing through the night while at school. "My reports started saying 'Chibs has gone off the boil, and not handed in her prep.' No one knew what I was doing." It took another six months of editing before the agent thought her book was ready to sell, but, within two weeks of putting it on the market, there were three publishers in the bidding.

The Spider King's Daughter tells the story of two young people who meet in Lagos. Seventeen-year-old Abike Johnson is the favourite child of her highly successful but brutal father. She lives in a huge mansion in Lagos, surrounded by armed guards, and is driven to and from school by a driver in a huge black jeep.

One day, she buys ice cream from a hawker, who lives in the city's slums with his mother and sister. Told in alternating voices, from their different perspectives, the novel charts their unlikely friendship, which evolves from its tentative roots into something more intense and complex, as a previously hidden connection between the pair emerges. The tension builds towards a dramatic conclusion.

THE picture that Onuzo creates of contemporary life in Lagos is one of a society acutely divided, in terms of wealth and opportunity, and riven by corruption - something she has seen first-hand in Nigeria. "The extreme wealth of Abike's family is not my background, although I went to private school.

"You can shut yourself off a bit, but the church we went to has people of all social classes. My father is from eastern Nigeria, and traditionally you go back to your village; so we have seen subsistence living.

"The idea came to me when I was maybe 12, and there was a competition and I wrote an essay about child labour. I interviewed a girl hawker. It struck me even then that, as she looked into the future, this was the only life that she saw for herself."

The hawker in her book is never named - he is just "the hawker" - which is, of course, a metaphor for his powerlessness. "In Nigeria, you have very luxurious housing right next to shacks and slums - it's a mish-mash. You can be insulated only so much. But Abike realises her life is poorer for her upbringing. She has never had a guide."

She defends Abike and her behaviour, which, from the opening page of the book, can be extremely cruel. "People need to cut her a bit of slack. Anything kind she does has to be multiplied by ten because of her childhood," she says.

"Abike has, as her background, a father who has trained her. She doesn't know how to be normal. She sees things differently. The hawker isn't perfect, but he's easier for the reader to forgive."

ONUZO grew up in a Christian home, and has kept her faith, except during her first year at university, which she admits was "a year of doubts". She says: "I was still praying and reading the Bible, and going to church. In the end, I had to make a decision: 'This is what I believe.' You ask yourself, 'How can you not believe?' That is faith."

She attends Jesus House, an independent church in Brent Cross, where she is an instrumentalist in the band. The week we met, her father, a medical doctor like her mother, had come from Nigeria to preach at the church. She is the youngest of four children, and is close to her two sisters and brother.

She came to England, along with the sister closest to her in age, to study for her GCSEs, and it took a great deal of adjustment. "My sister and I were two of only three black people in the school. I didn't like it for the first two years, but, eventually, I got used to it, and made some real friends."

She regards her writing as a God-given gift. "I believe God puts gifts in all of us. But that doesn't mean you don't have to work at it, or that you emerge fully formed. But if something is your gift, it is easier to work at it. It's easier for me to work at writing than at maths. When I feel discouraged about my writing, I pray about it. And, because it is my gift, I know it will work, no matter how long it takes."

Writing her second novel is proving a little easier than the first, "because at least I know I've done it before". She still writes through the night, and says that fitting it in around her studies has worked, because a history degree has fewer contact hours than many courses, and allowed some flexibility in her timetable.

SHE is uncertain what the future holds. "I do want to write more books, but I also want to do other stuff. I may move to Nigeria, and move into development, or politics. As a Christian, living in a country such as Nigeria and seeing everything going downhill, you feel 'This has to stop.'

"The thing is, in Nigeria, the money and resources are there. It's not about having grand designs, but about the money that is meant for schools actually going to schools. It's not about having a big macro-economic plan, but about doing the right thing. That's how I feel. It's about finding any way to stem the flow."

She admits that the situation gets her down each time she returns to Nigeria. "Then you soon realise being depressed is not going to help. For now, you pray. You pray God will use you to answer your prayers. And you hope God will give you grace to stand up for what you believe.

"But to go into politics, you need personal courage. Just when things are getting better, you hear that someone has been assassinated. My friends say: 'Why not start a charity, or an NGO?' But there's only so much you can do from outside government. As an NGO, you will always be on the periphery."

For now, though, there are graduation to look forward to, literary festivals to attend, and the small matter of the second novel to complete, before she sets about changing the world. "If it is what God wants me to do, then he will give me the courage to do it," she says.

 The Spider King's Daughter by Chibundu Onuzo is published by Faber & Faber at £12.99 (CT Bookshop £11.70); 978-0-571-26889-4.