Need help...
Sara,
First thing is what are you looking to sell, sometimes this can help you find what type of store you'll need.
Second, do you have a domain name and is it hosted anywhere? A lot of times the hosting company will have some e-comm setups or packages all ready to go and it makes it much easier to get setup.
Third, this one is what really hits home. How much are you willing to spend? This is the most important answer, sometimes setting up and e-comm site can add extra charges to your hosting fees. Depending on how much business you do, there are other expenses such as merchant accounts, some charge monthly fees, and others don't. Most merchant accounts always take a percentage of you transaction, so this is something to figure into your costs.
This is the real bare bones basics, let me know if you need more help.
big_kahuna