Good advice for starting a light novel?

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Don't know anything about writing novels but I respectfully disagree with Chrono. One should have a conception, at the very least a rough sketch of the entire story before starting. Inspiration can strike while writing and you of course should not resist that but to do everything as you go along is a mistake in my opinion. It is impossible to develop your characters and story if you don't have tight control over them. Before you start writing I'd recommend making a draft of an outline, letting it simmer for week or a few weeks and then return to it completely unattached. If you still feel it is compelling when the excitement has worn off, you should pursue it. If not, do another draft and repeat until you have something. Change plot elements or characters if you need to. The worst possible thing you can do I think is abandoning a good idea simply because you don't know what to do with it.

That's my two cents anyway. To recapitulate, I think it's necessary to know absolutely everything about your novel to be before writing if possible, because then instead of figuring out what happens as you're writing it, you can solve the more important issue of how to express it. Good luck.