One of the biggest tips I can give you is a lesson most of us learn the hard way:
Never give a low price based on a promise of future work. Usually these future jobs never materialize. An alarm goes off when a customer says something like "I've got a bigger/more project(s) I'll give if you give me a good deal on this one." These are leaches. They probably won't ever call you again or they always want a special low price for all their crappy little projects.
I also second doing everything above board. And don't undersell yourself, you'll just become known as the guy to go to for the cheap price. You have to make a profit. Profit is not your income, it is money above your pay that is to be used to help you grow and get through the rough spots. Speaking of money, you have to have some working capital unless you want to struggle for years just getting by.
If this is going to be more than just a hobby, I recommend reading a book called "The E Myth - Revisited".
