The affiliate game is brutal. Commissions start off low and usually don't get much higher. My advice would be to pick a niche market where you see potential for growth. Also - find out commission percentages before you dive in, factor in your hosting costs and marketing budget. Expect to operate at small margins. At least at first...
If possible, try to avoid signing up through a network like Commission Junction (they are essentially a middle man) and instead try to partner directly with the merchant. You'll find that you'll get higher commissions and you will be able to operate at better margins. Then again... finding a merchant, in a niche market, that deals directly to affiliates And supports their own program ain't gonna be easy. Perhaps it's just better to start your own business... And while you're at it, offer you're own affiliate program. :P