In most states you would need to open a business. Check with a professional if you want to see if there is a way to avoid opening a business.
The simplest type of business is Sole Proprietor, which is very easy and cheap to establish. You can even file your business and personal taxes as a single tax return in some states.
A corporation or LLC (limitied liability corporation) or s-corporation are more expensive options. Also these are MUCH heavier on paperwork and more difficult to manage.
The good thing about corporations is that there is a separation between yourself and the business. If your business gets sued or is unable to pay debts and you are a Sole Proprietor they can go after your personal assets (House, Car, etc.) If it is a corporation you are protected from personal liability (again, check applicable laws).
However, with a new business you will often find yourself signing a "personal guarantee" in order to get funding or to open a merchant acccount for credit card processing. This uses your personal credit rating to obtain the loan/account but it also makes you personally liable for any bad debts owed by your company.
My advice is to open a business of some sort. Once you establish a successful business, many doors of opportunity will open. Funding for new ventures comes easier, and you will get better terms on all your agreements.
I hope this helps. I am speaking only from experience and I am not an expert of any kind.