I suppose you could start by exploring a bit in your local area and find out any service/product you can provide for which exists a need. For some tips try visiting http://www.jatang.in
Presumably the point of the exercise is to learn sales and marketing skills so the obvious one to try would be to sell on Amazon or Ebay, where there s loads of ready made competition. I m sure you ve got items around the home that you no longer use so start with them. After that you can also investigate buying in stock on a wholesale basis or using a dropshipper (even if only as a paper exercise).Good Luck!