A program to track your hours, something to make a contact database and a todo list. All of those are pretty essential.

Billing and such you can do with any word processor.

If you go this route make sure that you reserve enough for taxes once the money starts coming in. Plenty of people fall into that trap. A good rule of thumb is to take the 'gross' tax rate and reserve that much right away whenever a bill gets paid on a savings account.

Then when tax time comes around you have your deductibles and you end up with some money left over instead of a hole to fill.