How can I setup ecommerce in my site, I want to start ecommerce

You will need an SSL cert possibly unless the electronic payment gateway has one that you want to use.

You need a merchant account and an electronic payment gateway or possible a third party processor and use their proprietary gateway.

What country are you in? That will help in answering the question.

You will also need shopping cart software.
It would be best of you tell us what kind of E-Commerce site you plan on running, so we can better help you.

Choose a webhost with cpanel hosting and use a shopping cart that comes with fantastico... just make sure it's secure

most of the web host support ecommerce