Good advice for starting a light novel?

[Froge]

I've written a complete novel before (50 000 words, NaNo). It sucked.

To start, the best way to prepare is to actually write one. Very few people get their first novel published. Pretty much the only way to write a good novel is to have experience. And you can't spend too much time preparing, or you might lose interest in your work by the time you've got everything figured out. It really shows in your writing if you aren't interested in the story anymore, unless you're one of those tank writers like George r.r. martin or Tolkien who can spend a lifetime on a series.