Once you figure out what you are selling Shopify (https://www.shopify.com/) is a good place to start with building your online store. They are offering a 90 day free trial at the moment to help our during this time. They have a lot of tutorials on how to get set up with things like Facebook and Google advertising. Once you get into it, let us know if you have any specific questions and we can do our best to point you in the right direction.