Only you can figure that out for yourself. Being successful at Affiliate Marketing heavily relies on the creatively and tenacity of the Affiliate Marketeer. Now for that to happen you should really love, or atleast have a good idea about the functionality or real life benefits of what you promote. That should make it a little bit easier for you to start earning through this model.
So to get you started, sit down, take a piece of paper and note down 3 things that you believe fit what I have mentioned above. See if you can think of a few topics related to the 3 things that you've made a note of, and gauge if you could put down content for them if you had to.
Also check if the topics that you've noted are topics that people are actively searching for on the internet. If yes, you know where and what to get started with.
I think you want to ask what sell best in terms of affiliate marketing. Well, for that you have to do some research. Everything sells on internet as far as i know. From clothes to medicine but usually people spend money on books - check amazon success. It started from books.