First, that depends where you live. Some cities/counties/states have special rules for things like this, and you'll have to get a health certificate in order to sell food to the public.
Getting that out of the way... how you prepare it depends on what you want to make. Sandwiches? Cookies? Muffins?
There's a lot of things you can make - just make sure you make it fresh and that it doesn't get soggy or anything. No one wants a sandwich with wet tomato in it making the bread soggy...
Got it? 