How can I know before I take action?

I need to have a way to know which internet marketing niche to go for before I take action. I saw this website [ http://seriousweightlossteaprogramme.com ] promoting a weight loss product called Lipo slim. I wonder if I just copy others or do some kind of research to identify the hot products to promote online! Please advice me and show me a way.

Thank you
Mila