Thsings to consider... 1. What is your interest? 2. Are you planning to sell physical products or digital? For digital products, refer to clickbank... for physical products, see amazon or ebay for best selling products..
You should get started on the market and target before the product. You can find more about this topic here - Conduct a Market Analysis If you want a step by step to start your own online shop then I would recommend you read here - How to Start An Online Store - eHow.com
First check product what you like. Check competitors in different products, your interests etc. After having product consider: dropshipping or wholesale buying. After that: check for producers/dropshippers on google, forums etc.
two steps 1, use ebay plus to find out which product is hottest 2, use google to find wholsesale seller of this product .