Most US states have rules that go something like this:
- You can register your business with the state. You'll need to choose your entity type (LLC, corporation, partnership, etc.), and the specific types vary a bit state to state, so you'll need to check what your state has.
- Registering will require an annual fee, so check how much that is and whether it's worth it given where you stand now.
- You'll probably need a "registered agent" in order to start your business. The registered agent typically has to meet certain requirements like have an office in the state where you register and maintain regular business hours. They will receive official notifications from the state (like when you need to re-register), and, in theory, if you ever got sued, the service should be on the registered agent. You can be your own registered agent, although there's different schools of thought on whether this is a good idea. If you're working out of your house, for example, it's probably not a good idea because your home address will go in the public record and you probably aren't keeping regular office hours. If you're going to have a store front somewhere, then maybe it's ok. There are companies that are in the business of being registered agents for ~$100 - $200 / year if you want to out-source it.
- Depending on how you organize, you may or may not need anew federal tax ID. If you're a sole proprietor, for example, for federal taxes, you'll use your personal social security number. If you're most other types, then you'll need an employer identification number (EIN), which you can easily get online from the IRS once your business is registered at the state.
- Once you have your business registered with the state and have a tax id (which may be your SSN, see bullet above), then you can take your incorporation documents to the bank and open a business account.
You will almost certainly have to pay some registration fees, as noted above. Depending on how you organize, you may or may not need to file a separate tax return for the business. (If you're sole proprietor for tax purposes, then you file on Schedule C on your personal Form 1040.) Whether or not you pay taxes depends on whether you have net income. It's possible that some losses might also be deductible. (Note that you may have to file a return even if you don't have net income - Filing and needing to pay are not the same since your return may indicate no tax due.) In addition, at the state level, you may have to pay additional fees or taxes beyond income tax depending on what you sell and how you sell it. (Sales tax, for example, might come into play as might franchise taxes.) You'll need to check your own state law for that.
As always, it could be wise to get professional tax and accounting advice that's tailored to your situation and your state. This is just an outline of some things that you'll need to consider.