A Limited Liability Company would probably be best suited to you. Like DA said, if someone sues, they can only sue your company and would not be able to go after your personal stuff. If you filed a corporation, they could take you for everything. LLCs are the most popular entities right now. I work for a corporation supply company and the majority of the books we do are for LLCs.
LLC's do not guarantee that you cannot be sued personally. They do generally make it more difficult, but LLC's, just as corporations, fall under state (not federal) jurisdiction, so the laws pertaining to them vary from state to state.
The US Small Business Administration cannot advise you which legal form of organization is best for you - or at least is not supposed to. (I am retired from SBA). They can only provide you information about each form of organization.
As a reminder, giving legal advice to someone is against the law unless you are a licensed attorney.
You can investigate the different forms of organization and decide yourself, or as I said earlier, seek advice from a CPA, or better yet appropriate legal counsel.
The primary matters that need to be taken into consideration when selecting a legal form of organization are the type of business, exposure to liability issues of the business, the amount and nature of your personal assets and the assets you are likely to accumulate, your personal current and proposed tax parameters, and the corporate and LLC tax and liability laws in the state in which you live.
OP, as to the other question you posed, generally speaking you only have to be licensed or registered in the jurisdiction in which your primary business is located, so normally you aren't required to have a license in other towns you might work in unless you maintain an actual business location there. You will need to check with your county clerk, and city clerk if your business will be within city limits, and check your state website for any state requirements...