If you want something quick and easy to spin up, just set up an LLC in your state of choice. Set up an accompanying business bank account(if you will be processing high volumes of transactions look into the per transaction fee structure). If you will need to collect tax in your state, you must of course, file with your comptroller.
If you have multiple members of your LLC, set up an operating agreement. Set up a Federal Tax Id, i.e. the Social Security Number for business, aka EIN. Consider if you want business insurance.
I think other things can fall into place. Start recording books from day 1. Easier than it sounds, and some of those things are optional. There are a lot of 10 step guides online. You can always change structure later, LLC -> S corp, or C-Corp.
Make it as frictionless as possible for yourself now to keep momentum.
