That's really, really broad. Are you selling your own stock or drop shipping? Be prepared for shoplifting/chargebacks and factor that into your pricing. Will you be employing staff? Make sure they're trained and have appropriate permissions. There are tons of plugins for systems like woocommerce so there's no one way to run your business. You'll need to go through them and decide which you need, and which you need to buy. You need to be checking emails or enquiries at least 2x a day. People get impatient and unless your products are really special people will just buy elsewhere or buy something else.