You don't need a license to do advertising or any sort of public relations, but you do need a general business license to protect yourself and make sure you are going to report your earnings properly for tax purposes. You can get a business license at your local county auditor's office--you pay a fee, register your business and get a EIN number (tax reporting number.) You can be your mother's business manager, and then expand to other businesses--good idea! Keep careful records. And you should draw up a contract form for your clients to sign that states clearly what services you provide, what fees you charge for these services and how and when you will be paid. Other than that, you don't need to worry about licensing or permission--any contract your clients sign gives you the permission legally--and if they agree to it, you have legal standing in case they decide not to pay you.
Good luck! It sounds as if you may have found a home business niche that will work out pretty well for you!