If you didn't know about Clickbank, you clearly didn't do very much research on affiliate marketing...
I agree with AJP Peon I would also choose those products which I would have good knowledge, a familiar touch and believe on that product. And yeah search also, which products are in demand nowadays.
You don't have to actually join some affiliate network to be able to see which products they are offering, it's especially easy on larger networks. Or you can research what's popular by following some blogs or pages related to trends, as you will find a lot of useful information there.