ealdgyth
I was in a similar position to yours when I started here just over a year ago. It's like xiaohai_23 said: you just start writing, and keep on writing. Post new chapters frequently to get more readers quickly.
You'll be using Inkstone - that's the in-house application here. You probably know already that you access Inkstone by clicking the Create button at the top of the WN main page. It's pretty intuitive, you should get the hang of it after a few minutes of fooling around. You can write using Inkstone or whatever app you prefer, then copy and paste the text into Inkstone.
If you run into technical problems, I'd recommend contacting support instead of asking for help on the forum. They've always been very helpful to me.
Writing a fiction serial here differs from writing for a traditional publisher. You'll be posting what is essentially a first draft with some very light editing: punctuation & grammar errors, rephrasing things here and there, that sort of thing. The vast majority of people who write here don't even fix their typos. BTW, you won't be able to use bold or italic font if you aren't a WN-contracted writer.
WN requirements? There are no requirements per se. Same goes for community expectations. This site is mostly about encouraging young people to read and write, and that's what I like about it. Needless to say, overall the literary standard is very low. Quite a few people have raw talent, but next to to none have the skills needed to produce good prose. One of the more popular writers on this site stated that it takes 'six months to write decent (sic)'. You get the idea.
For me personally, writing here is a nice way to unwind after the frightful commercial shit I have to write in order to earn a living, and that's the approach I'd recommend to another pro writer. One more thing: don't plan to get rich through WN. Most top writers here make as much money monthly as you can earn in a single day writing advertising copy for a client with deep pockets.
Good luck and bon voyage!