I agree with the comments and advice given so far. I'll add a few miscellaneous thoughts:
1) Contact the Small Business Administration or other resource for literature or assistance in developing your business, even if you're not going to use 100% of their services. Search the web, go to the library, talk to others in your area about both the errors to avoid and good advice that will give you any advantage in your business. Then ask each person, "Are there any questions I should ask but haven't yet?" and you may find they offer a true gem.
2) Do the difficult work ahead of time, as suggested, to determine your minimum rate and make sure it includes that retirement benefit mentioned. Establish a rate, compare it to your competition and then never feel guilty about what you must charge, especially if this is the livelihood that will feed, clothe, protect, and educate yourself & your family.
3) Do get organized ahead of time. If you can locate or develop standardized forms for any quotes, contracts, design, invoices, statements, etc. It will leave a much better impression that you know what you're doing and how to get it done accurately. They don't have to be elaborate but they do need to be accurate and complete.
4) Be realistic in your planning. In your business you may need a year of 'try it and see' before you know whether a plan, a rate, or a process will work out over the long haul.
