Each state will have different requirements so you should first check with your state's website. Some have "How to do business in xyz" pages that are very helpful. The type of legal entity is not too important if you have no employees and have a relatively low volume. An LLC is more complicated and has a cost involved, a sole proprietorship is simpler and has minimum costs. An LLC provides in some cases protection that a sole proprietorship does not but if you are the only employee then it can get a little fuzzy. Either will work. Also check if your county/township/city also requires a business license. Some do, some do not.
If your state charges sales tax, then you will likely need a state tax number and be required to charge, collect and remit sales tax. But check, not all states are the same.
Make sure your "business name" is available, that information can be searched on at your state's website. If available, you should register that name ASAP. Likewise make sure any domain names that you will need are available and register those as well. You do not want a legal business name that cannot be used on a website because someone has already registered it.
Talk to an accountant/bookkeeper and possibly a lawyer to get things setup. Think about how you will handle bookkeeping, payroll, banking, receipts, credit cards. Have a separate checking or Paypal account just for the business. Start to think about how you will separate business expenses and personal expenses. The business is a separate entity and must be treated that way. Always buy business stuff with the business's money and your stuff with your money.
Insurance is extremely important if you will be doing portraits, your home owners will likely not cover you unless you specifically tell them you are running a business (assuming you are working from your house). Your equipment will also (possibly) no longer be covered by your homeowners insurance since it is now business equipment. You will need business insurance, equipment insurance and most likely a personal liability policy to protect you from law suits.
---------- Post added 06-14-14 at 01:49 PM ----------
This might help:
Sole Proprietorships vs. LLCs | Nolo.com