Help getting started with an e-commerce site

Hi,

I have just installed wordpress for use with a new domain i bought.

My plan is to plan is set up a store, promoting affiliate products to begin with, but quickly moving to sell my own.

MY question is, how do i best setup wordpress for this? Should i install an e-commerce theme, or just use a plug-in?

Being completely new to wordpress, which simple (free) e-commerce themes or packages should i be looking at?

I plan to pull the data feed from my merchant.

Thanks