Here is my advice.
1. Start with an attorney, or legal assistant, to help formulate the business structure. S-Corp, LLC, etc. This is the critical first step. You have to structure right early on to make it easier. Some accountants can help guide you hear as well.
2. Find a shopping cart solution. I would suggest if you want to go cheap to try ecommercetemplates. Very flexible, affordable and allows you to do your own customizations. Another good choice, but a little more costly, is AspDotNetStoreFront (however that cart is going downhill lately).
3. Now you need to line up merchant services and gateway. Talk to your local bank, they can probably help. Search for an authorize.net gateway reseller, they can help too.
4. Find a rock solid host. I can suggest the best in the business if you are on a php cart (ecommercetemplates is php) and I have a good host also for .NET carts. PM me if you are interested in who they are.
5. Work with a web designer to develop the look and feel of your cart. This is critical, you want a sharp, crisp, easy to navigate user experience.
6. Get your shopping cart deployed to your server, get merchant services hooked up and start testing. Test, test, test. Act like the dumbest customer on the planet and test like that customer. You want a ROCK SOLID shopping experience.
Those steps will get you going as far as the backbone of the business.