It is completely legal: any person may offer to teach anything (well, anything that is not itself illegal--e.g. no classes in bomb making or child pornography) to anyone else for pay. Certainly, there are times when a credential on the part of the teacher is needed to grant a credential to the student--a  layperson hold classes in the law, for example, but not grant a law degree--but that's a different issue. Do make sure you don't misrepresent yourself as having any credential that you don't.What you need to make sure are the same things that any business needs to make sure of: that you are properly registered (and ideally, you should set up an LLC or corporation to limit personal liability), that you have appropriate insurance, that you pay the appropriate taxes, that you use a business name that is available for use, etc.