Determine the type of paint it was painted before. Some paints will peel if applied over others. In that case you have to use the same base paint or remove the existing paint. scrape it. Don't paint over wallpaper also. Other then that get a brush or roller and go to it. After you've washed any dust off the wall. Patch any holes with a putty.
so when i painted my room i 1. taped all areas where my wall touches other stuff (i.e. doors, windows...) 2. lay down newspaper on the floor (so i didn't ruin the carpet) 3. bought paint and primer (primer covers up the other color and makes sure the paint sticks) 4. bought a paint roller, paint tray (where you roll the paint on the roller) and a smull brush for corners and sides. 5. PAINT!!!!! (Maybe with some music?)