If you don't already have a product and are looking to start an online buisness, this is one of the most important steps.

Know the market -

• What other sites are selling your product?
• Where are your competitor's getting their products from?  If they are coming from the same place you plan to get yours, what are you going to do to make them buy from you?  Can you compete on price?  Advertise better?  
• What will your markup be?  Can you sell enough  of your product to cover your cost?  Can you sell enough to make it worth the time and effort you are going to put in?
• If you plan to do most of your advertising on search engines, you have to know what your competitors are paying per click.  Is it more than you can afford with your markup?  (Keep in mind that a very small fraction of the people that click on your ad will actually purchase from you).

• Dropship vs. inventory  (What is dropship?  Why should I run from turnkey dropshipping solutions?)
• Online store vs. ebay