You can start by reading around the boards. If you're not sure how to do something, you can learn a whole lot from other writers who can.
We have a built-in spellchecker (the button is right next to the 'post' button). Use it. Even good spellers can have a dumb now and then.
Make sure you discuss thoroughly with your prospective partner before the game actually stars, to see if you are compatible. If your literacy levels, post length and frequency don't match, you will both be disappointed and put off, whether that's in PM or in open forum.
It can be daunting to write in the open for the first time, but keep in mind that most people are too busy with their own stories to trawl around for other people's stories to bash. Sometimes, though, you get a PM from someone who liked one of your stories, and that makes everything worthwhile.