WA's mentioned The Spuffy Realm but there are loads of other great archives: Elysian Fields, the Bloodshedverse, Nocturnal Light, Sweet Temptation, Sinister Attraction, All About Spike...
There's also a huge active Spuffy community over on livejournal. A good place to start might be seasonal_spuffy - a lot of the currently active members of fandom participate.
As for writing fic I agree with WA - start small and get yourself used to writing the characters, setting etc.
When I started writing Buffy fic I made the mistake of trying to do the big, epic fic straight away and I had to give up after the first five chapters. My next attempt was a one-shot and it was much more successful.
That's not to say that some authors don't get the novel-length thing right first time, there are lots of them out there.
My biggest tip would be to try and find a beta reader. A good one works wonders!
Good luck and share a link to your fic here if you write it. 🙂