There is no legal requirement that you form an LLC (or other entity) to start a business. But here’s the $64,000 question: Do you want to have your personal assets stripped from you? Do you want to be at risk in the event of a lawsuit? Do you want to spend a lot of time and money building a business that will always be dependent on you?
If your answer to any of these questions is “No,” then you should form an entity. Take this seriously. Sketch out a business plan. Project your expenses. Don’t assume that the Internet still operates on a “build it and they will come” mentality. (It doesn’t.) According to Mashable, there are 350,000,000 registered domains, and 150,000 more are added each day. That’s a lot of noise and a lot of traffic to cut through.
If you’re not serious about running this as a business, then figure out how much you’d have to spend on it and put that money toward something else.