Sit down and just read the stories, read the canon. Give yourself time to see the lay of the land before you start to buy a tractor and seedlings.

The most important thing to do in fandom is not be afraid to write stuff that you like. The only reason I started writing fanfic was the fact that I didn't find stories that I liked, so I wrote things for myself. Having people liking stuff is a bonus, but if I don't like it to some degree, I won't post.