Aruna7 knows more about Star Wars than I do. She probably knows more about Star Wars than any other person I've met, and she may know more about Star Wars than George Lucas on any given day. When I first started writing Star Wars fanfiction, I was uncomfortable because I wasn't as well-versed in the GFFA as I would have liked. I did as much research as I conceivably could, and then I started writing, relying on my friend to help me fill in details and check my characterization.
There are other fandoms where I feel (or have felt) more confident. Stargate SG-1 is a good example. (I know, I keep coming back to those two fandoms, but that's because I've spent most of my fanfic writing time in them.) I'm sad to say I've forgotten a lot of my detailed knowledge of the canon now. It's been about five years since I stopped writing SG-1 regularly, and my memory is fuzzy.
Even before that though, there were people in the SG-1 fandom who knew things I didn't. It was great to have them around, but sometimes it presented challenges. Fans get nitpicky. Star Trek fans are known for being especially bad about this. People can be rude, and you will find individuals who have nothing better to do than knock your story because you forgot a detail or made a choice they didn't like. They forget the time when they were new to the franchise and didn't know things either. Sometimes I ran into people who thought they knew something and knocked my story unfairly.
So, what I've learned is to do my homework until I'm comfortable, then just go for it. You'll never really know "enough" about a franchise--and you'll probably forget things anyway. You'll continue to learn more as you go. Fanfiction should be a labor of love, not a trivia contest. If you let yourself be intimidated or put off by what you don't know, you'll cheat yourself and anyone who might have enjoyed your story anyway.