Schedule some time with a trainer, or if you are interested in a particular plane, ask around for someone who knows it. My best advice is to listen to the trainers first to avoid getting into bad habits. Read, read and read some more. Compare the performance charts so you know what each plane is capable of. The toughest things to learn IMHO are Situational awareness and throttle/speed management, or at least they were when I was learning.
F6F is, as others have said, a fine plane to learn on because it has few vices and is competent, if not stellar, at almost everything. Learn which planes you can out-turn and which ones you can outrun, and fly accordingly.
Finally, work on figuring out a good convergence that works for you. There's no real good answer to that...basically you need to set the guns at the point where you are most comfortable taking a shot. I usually ping at about 400 and go for the kill at 200, so I find 300 works best for me.