We checked with Lowe's home improvement stores and they calculate that 1 gallon of paint should cover about 350 square feet.
If you also intend to prime the area, 1 gallon of primer will cover about 200 square feet.
Naturally all estimates may vary depending on how you apply the paint/primer, and even the brand can alter that estimate, so it's always a good idea to check on the estimated coverage for a specific brand and job.
To help get you started, RealSimple.com offers their 6 Simple Steps to Painting a Room, HowStuffWorks provides tips and guidelines on How to Paint a Room, and Benjamin Moore offers Painting Techniques & Tips - How to Paint a House or Room.
In "ahhh, a happy place" news, the Oregon Live is reporting that A new coat of paint can lighten up the smallest, darkest room.