1. Try out colors before painting your walls. Almost every company offers sample sizes so you minimize buying and then having extra paint around. Don't test by painting directly on your wall. Use a product like SMALL WALL so you can move samples around your room (into the corners and in dark and light areas) and note how the color(s) looks at different times of day. You'll save on priming, and being able to separate two similar colors to determine which is best is priceless! We carry them in the shop (www.tot-home.com) and Ace Hardware and Sherwin Williams also sell them. 
2. If you have doubts about which color to select when comparing 2 similar colors, go lighter. You're looking at a small scale on a chip or board, and when on the walls - especially inside a small space - the impact will be significantly greater.
3. The right paint brush or roller is as critical to a good paint job as is the quality of your paint. I recommend getting the best you can afford and I use Purdy. A good brush - cleaned properly - will last through many projects. For oil-base paints, China bristle paint brushes, which leave few brush marks, are a good choice. For acrylics and high-quality latex paints, nylon paint brushes are best. Nylon-polyester blends and 100-percent-polyester brushes work with any paint. Expect to spend at least $9 for a quality 3-inch paint brush.For the best paint application, select a roller cover with the appropriate nap size. The label will guide you as to use on smooth or rough surfaces.
4. To buy the right number of gallons of paint, use this quick estimation. 1) add up the widths of the walls, 2) multiply by the room's ceiling height 3) divide by 350 (the typical square footage one gallon covers). The last number is roughly the number of gallons of paint you need. So, an 8' x 10' room with 8' ceilings would be 8+8+10+10 =36 36 x 8 = 288 288/350 = .8 or rounded up = 1 gallon You're not deducting for windows and doors, so you should have plenty of paint for touch-ups. If you use a sprayer, a gallon covers about 250 square feet.
5. The most important step is good surface preparation. Ensure a long-lasting paint job by taking time to assess the condition of your home and then wash, scrape, sand, caulk, and treat stains as necessary. TSP is a great cleaner for dirt and oils. Use primer to cover dark spots, existing paint colors, and to cover areas you've repaired. A fix now can save multiple re-paints later. Tape and mask trims, windows and more. You'll be happy that painting goes faster and cleanup is minimized.
Have fun!
Have fun!