Remove the outermost screws from the case - the ones that are holding the side panels to the rest of the case. You will likely need a phillips-head screwdriver to remove these screws.
Set these screws aside. You will need to use them to secure the side panels to the case again when you are through working inside your computer.
Note: Take care not to remove the screws that are securing the power supply to the case. These screws are more inset than the case retaining screws and may cause the power supply to fall into the computer, possibly causing damage.