Hi there, I am personal trainer. Working as one most clubs; if not all, will require you to get certified.  The certification process is extensive but well worth it.  Aerobics Fitness Assocation of America (AFAA) is well known and there are others.   I am employed, but also train on location and at client's homes.  This source of income is considered 'supplemental' for tax purposes and I pay taxes at the end of the year or quarterly.   Best of luck!

Welcome to the Community! Take a look at Business.gov's Start a Business guide first. Once you choose your business structure (step 5), you should take a look at Filing and Paying Small Business Taxes page for a breakdown if how to file taxes based on different filing structures. Good luck on your new venture. Stop by the Community again to let us know how you are doing or if you have any more questions!