just join a couple of them (copeac, azoogle, neverblueads, CJ...etc) browse the offers available and see which which will work best for you.
Experience. And the best way to get some experience is to give a couple of networks a try, and see what works. Good Luck! nscdn
There's no best niche or best product. There is only best marketing strategy for the product ot niche you'd chosen. And by the way, you may want to check out my signatures to see if you feel comfortable promoting adult material.
Check out the networks in my sig. They are all reliable. Any questions, feel free to ask. I'd be happy to help.
CJ is big but it does not work out the best for many of them. Try Ebay as you will have a variety of items to promote and good thing is that many of them are used to buying from eBay or have heard a lot about EBay. If they open a new account and activate it, then you will make $25 commission. OR Choose a niche and try out couple of affiliate networks to see what works best for you.