If you just want to play multiplayer, download the client!
Please note:
Download the SA-MP client installer from one mirror and install the program in to your GTA San Andreas folder.
Some SA-MP servers operate on version 0.3.DL with downloadable content. You need to download client 0.3.DL to connect to these servers.
You can use Open.mp Launcher as an alternative to SA-MP Client (you still need SA-MP installed though).