Well, you really have to be at least decent at fixed-media stuff (a.k.a. RL art; drawing, or painting, ect..). If you're no good there then you will have trouble with pixel art. If you are good there, it's really not a great leap to start doing line-art, and then shading, and ect... those things are fairly self-explanatory and can be done even in paint.. (note that this is coming from somone who has never really done any pixel art...so, all you pixel artists out there, if I'm wrong in anything, please feel free to correct me :P)