Carole, you raise an excellent point that should be included in this list. Any contractor coming on to your property should provide you with a Certificate of Insurance from his agent showing evidence of Commercial General Liability and Workers Comp. Without these protections in place, an attorney for a contractor's worker or a 3rd party injured in the course of the work on your property will name YOU in a lawsuit. It's a minor inconvenience up front to ask for this documentation compared to the cost of litigation when something goes wrong.
Before speaking to a pro, consider how many rooms you would like to paint. Ask yourself if your project will be limited to the walls of the rooms or if ceilings and trim need to be refreshed as well. Since standard equipment is used to prep a work site and apply most finishes, it's usually more economical for you to have multiple rooms painted at once.
To get basic measurements, measure the length and width of each wall you want to paint in inches. Multiply the length and height together, and divide by 144. This will give you the square footage for each wall. A gallon of paint typically covers around 400 square feet. In some cases, you may need a primer 2 first, then a single coat of paint, meaning that you may need one gallon of primer and one gallon of paint to cover 400 square feet. However, if the wall requires multiple coats, this will increase the total amount of square footage required, and therefore the amount of paint.