First of all you will need a good product to sell. You can sell bad products but if sites review your product, your sale will decrease. Secondly you have to make a very good commercial text to convince your potential customers. If the text is not good enough, no one will buy your product. Thirdly you have to get your product on the market. Make a good adwords campaign with good and relevant keywords. Use affiliates!!!! This is the most important thing to do. Make a clickbank account and just get yourself as many affiliates as possible. Let me explain to you why:
Lets say you're selling an ebook. Say you will get 200 affiliates. The price of your product is 30 dollars and your affiliates will get 10 dollars if they sell your product. 50 of your affiliates have sold your product once within two months. It doesn't sound much but you will get 50*20 = $ 1000,- ! You won't get rich with $ 500, - a month but it is a beginning.