There are 2 methods of painting,- the official"RIGHT" way, which you should always use if you own your place, or are planning to stay for more than a year. There is also the "CHEATERS" way which is perfectly appropriate for dorms, squats, stage, and cheap apartments. Both ways look similar, but the proper way is smoother, lasts longer and will save you maintenance in the future. In this Instructable I am putting in all the proper steps and will put in italics any parts that could be skipped if you're in a cutting corners mood.
Step 1: Choosing your paint
Here are two links that can help you find out how much paint to buy.
What finish do you need? A shiny finish is easier to clean dirt off of, but will show more imperfections. A flat (or non shiny finish) will cover up wall problems better, but is hard to get clean. The average wall is painted with an eggshell finish, sorta in the middle. Woodwork is commonly painted with a high gloss, which is very shiny. From flattest to shiniest the types are: flat, eggshell, satin, semigloss, high gloss, and enamel.
Primer? Quality? If you are making a color change you will need several coats of paint. A higher quality paint will cover better, meaning less coats. Primer is useful as a first coat when you are making a severe color change. It comes in white but can be tinted. Primer is also a good idea if your wall is in bad shape to help your paint adhere and last longer.
What type of paint? For indoor walls, there is no reason to use oil base paints. water based paint, i.e. latex or acrylic is fine. They are now marketing "greener" versions, that claim lower odor and better for environment . I have been told they don't work as well, as well as costing more. A more environtmentally concsious choice is to acquire "recycled paint" from your local Restore, or recycle outlets.