There are many products you can promote in order to earn money. One question you have to make to yourself, is which niche you are ready to promote well, you are familiar with and lot of people are interested on it.
If you are going to sell something that not many people like, you have less chance to make lot of sales. On the other hand, if you sell something that many people look forward to have/ learn.
Let me list some:
Body Building (everyone wants to be attractive)
Loose Weight (many people want to loose weight, especially now that is summer)
Dating (most of us are looking forward to learn a few more tricks about dating)
Stop Smoking (almost everyone who smokes would like to stop it)
Building abs fast (abs is something that make you look attractive and everyone would like to build them fast)
Those are a few niches which can help you generate sales easier, it doesn't mean that you cannot success with other niches.