First off, you're going to need rockstar's famous game named "Grand Theft Auto San Andreas" for the computer. Then, you're going to want to go to SA-MP.com. Then click "Downloads", choose a mirror, then click the desired mirror, and it should open the installer. Then go through the installer choosing wisely. Also you must have GTA SA installed before downloading SA:MP.
Info:
- Some OS don't allow SA:MP to work, some Windows 7 OS allow you to play it though
- SA-MP.com is the official forums, and webpage for SA:MP
- SA-MP is a mod made by GTA fans