I agree with Tink. First off, you need to have a decent camera, and if you don't, it's likely you won't get very good pictures. However, with adequate natural lighting even my camera can get a good shot of my fish. So make sure you have a really bright room and turn off the flash, and the higher the shutter speed, the better. Bettas typically move a lot slower than goldfish, but if you want that perfect shot, make sure it's FAST.
I find that when I photograph my fish, even with my boyfriend's really nice camera, it takes a LOT of photos to get one good shot. In my last "photo shoot," I took over 200 photos, kept about 20, and only thought about 8 were good enough to warrant sharing on Koko's.
Good luck!