Step 1:
Decide exactly what you want to paint--interior, exterior, and how much detail. Take reasonably careful measurements of wall height and length, the amount and size of baseboard, and number of windows and how much detail will need to be painted for each window.

Step 2:
Take these measurements and use them in one or two of the calculators available on the Internet (see Resources below). A rough guesstimate is 300 square feet per gallon of paint. Allow 2 quarts as a reasonable buffer.

Step 3:
Shop for the best price on paint and availability. The major discount houses are now very competitive and have large inventories. Don't discount the value of finding a vendor with an employee who is knowledgeable.

Step 4:
Take your estimates to your chosen vendor, but allow them to make an independent estimate given a discussion of your circumstance and needs. If there is a huge discrepancy between the two estimates, figure out why.

Step 5:
Make sure that the number of coats you have estimated to be needed is appropriate before going too far into your project. This should be possible by painting a small area. If you need two coats instead of one, your estimation will be just a bit low.