for a slightly more advanced look at 'playing on a forum' I can recommend...
http://wiki.tf-575.com/index.php?title=Play_by_forumIt's written by a guy in a friendly group, which has a few of it's own over here too. The writer is heavily prejudiced against play by email and the like, but the core of posting conventions is probably useful.
The main thing though, as was hinted at earlier... get in contact with the person running the sim you are interested in, realise that you like this person and that they are friendly and approachable, then talk to them about what they can expect and perhaps even offer them the chance to help.
If you do 'muck up' as you seem to be worried about doing, only the very worst of sim leaders are going to throw tantrums, and I honestly don't think there are any of them here. The worst that can happen is that you'll get a friendly message from one of your new mates suggesting things you might want to change.
So.. dive in, give it a chance.. I am confident you'll like it
