Use wood that will withstand the weight of your computer and other heavy things on your desk. Think about the type of wood. Some wood expands and shrinks with humidity. Two big sheets of wood from your local hardware store should do it.
For a basic desk, cut one of the sheets of wood the dimensions you would like the top of your desk to be. Then cut a second piece to be one of the sides. Thirty inches high is fairly standard. You will need to cut into the second piece of wood for the second side. Take a leftover long piece of wood, and cut a support beam. The beam will be measured based on how big the top of the desk is.
Lay the top piece of the desk down. Take one side panel, and attach it about 4 inches from the edge with the corner bracket. Repeat with the second side panel. Now attach the support beam to each side panel with the corner bracket. It should be about 2 inches from the back edge of the desk. Turn the desk over and paint it as you wish. Be careful because some paints cause interference with your computer mouse.