Best thing you can do (this is with any business) is figure out who your clients are:
1. Where do they live
2. What age are they
3. What is their expected computer knowledge
4. How much can they afford to pay
Then you will know where to advertise, what to say on the advertisement, and how much to charge. Your performance will ensure a word-of-mouth growth in certain age groups (especially older people).
My suggestion is to target the elderly as they usually need the most help, are willing to pay for it, and will tell all their friends. I would also suggest offering training (how to email, how to send photos, how to...etc.) Also, get a Sprint phone (since they allow free incomming calls now) and have that as a dedicated business phone.
As for the legal stuff, get a Nolo press book (they put out tons of "how to start a business" books). They will have all the paperwork, and instructions you need to get started.
Best of luck!