A business license is easy to obtain. Contact your city hall or chamber of commerce to get the paperwork you need to submit-- and it's just better to have one sooner than later.
You don't need a tax ID#-- you can use your social security number if it's a one-person, small business. I would go and talk to an accountant (most will meet with you for free). You will want to set up a business account for yourself at your local bank, with your name Doing Business As (DBA) Your Business Name. Keep all the money you earn there, and then "Pay" yourself into your personal account from that account. Then, you can see clearly, at tax time, your earnings.
Also? Set up a savings account. Whenever you make ANY money, put 25% of it immediately into your savings account. Because that is how much you will be paying Uncle Sam, or Uncle Canada, depending on where you live. Your accountant can also explain things like quarterly estimated tax. It doesn't matter whether you make $100, $1000-- you still need to plan for taxes.
Do you have any other questions? What is your business going to be?