Not sure what state you are in but more than likely your services (assuming personal training) are not taxed. You will however have to pay income taxes on your income, as with any money you make. If you are selling products, whatever merchant account you use will calculate them for you based on the state. You then have to pay taxes quarterly on your sales. I assume you have a business license... if not, get one before you start to do anything. Not only is it the law but more importantly it protects you. Otherwise, YOU are liable.
I've never used paypal but there are lots of merchant accounts that you can use. Amazon has one and there are tons of others.