You aren't required to incorporate at all (but in many cases it's the best course for mitigating risk). People start businesses all the time without forming a separate entity. However, in the eyes of the law, that means that you and your business are one and the same. So if the business does a Bad Thing, you as an individual are responsible for all of the consequences of such Bad Thing. Similarly with taxes, all of the business' income gets attributed to you as an individual. Most people don't want these two things to happen, so they form an entity. You can go without, but it comes with significant risk, and you may end up forfeiting favorable tax treatment as a result.
These same issues get more complicated as soon as you involve others. Talk to your accountant. And consider what your risks are and how much your business is going to make. Find legal assistance for small businesses, or talk to a business lawyer as soon as you can.
If you are in the US, you can just use your SSN and sign up directly with your payment processor as a sole proprietor.
If things take off, you can start worrying about talking to a lawyer. You really won't have any reason to form an LLC until you need to hire real employees (W-2 employees).
Although, as has been said, you don't need an LLC if your business is not "there" yet.