rosi32 You don't have to pay anything to write. Just go to the 'Create' on the browser(main page of webnovel) and star writing! As for the contract, you have to at least have a lot of readers and maybe 400 collections in a month (this, I'm not clearly sure about it).
Another possible chances for you to get a contract:
1. 50,000 word count (or you can only have 15k word count and the novel is very popular then,you'll have higher chances of getting a contract.)
2. Stable release rate.
3. You can simply join the recent WPC to get a quicker and surer chance to get a contract. (for as long as you win the contest or at the top ranking.) (And based on my experience, this is more legit and faster.)
4. You have to at least rank within the 350 rankings,
5. And, it really depends on Webnovel if your novel catches their attention and offers you the contract.
I'm not sure about this, but that's everything I know that can answer your question.
Anyway, best of luck!