You can do ecommerce in various ways. However, here I am talking about two methods that cost little money to start.
Affiliate marketing: If you want to do affiliate marketing, all you have to do is set up a website, join affiliate networks and promote the products. Cost involved is domain registration and hosting fees.
Dropshipping: Create a website, join drop shipping companies, sell the products, forwards the address of the buyer to dropshipping company. Rest of the work is done by the dropshipping company. Sometimes you might have to pay for the product in advance.