create a page where you want your store
add an instance of the WebStore feature to the page
use administration links in webstore shown on the page to
but you can't sell products directly, they are sold through Offers, so you have to then create offers and add products to the offer and set the price. you can have a 1 to one relationship between products and offers but you can also have multiple products in a single offer, ie you can sell and album of mp3 files
once products and offers are defined and enabled for sale the instance on the page will show them and there will be a cart link on the store page (there is a way to put a cart link on all site pages later after the store is working)
WebStore is mainly good at selling download products, there is no support for separate shipping costs.
In order to work it also requires a payment gateway to be configured or paypal to be configured
you can also poke around on our demo site where you can login as admin and we have a store and products all set up, so you can compare the way things are setup there with your own site