If this is your first time marketing online, I wouldnt suggest you start with big niches like weightloss, acne, and addiction. You would need to start with a smaller niche and then move up from there...
like with computer repair... teaching how to use a program by creating video tutorials, or find something you know how to do well, create a product and then promote that online.
There's an article by russell brunson that will help get started :
Dead ducks don't quack | Internet Marketing - DotComSecrets
If ur really interested in getting started i'd suggest his free dvd... russellsfreedvdoffer.com its what got me started and this summer, both my husband and I quite our jobs and now work from home.
If ur interested in getting started I have a series of videos I'd be willing to share with you that will take you step by step on how to set up ur own online business...
Best of luck,
Chaunna