how to open a online shop?

I'm not really sure. Have you looked in to ebay? I know sellers have their own stores on line. Could be worth a look. Good luck.

A lot depends on what you're after - are you wanting to sell your own goods or are you talking about basically being an affiliate? If your own good/services you can probably set up your own website etc. Which you can do for free For affiliates you can do things like an amazon store - freestore club etc.