To do this you can join affiliate programs of individual merchants, or signup with affiliate networks which allow you to promote products from multiple merchants with a single affiliate account. Here are a few popular affiliate networks you could be a part of to start with:
As a reseller you will earn an income from the difference in price of whatever product you buy and how much you sell it for. The product could be anything from infoproducts (e-books) to Consumer Electronics to apparels, and more.
Here are a few trusted wholesalers you can start with: