First off, you need Grand Theft Auto: San Andreas, which can be purchased on steam or your preferred Game store. After successfully installing the game, you will need the San Andreas:Multi-player Client(SA:MP) which can be downloaded from
here. This Client should be installed in your GTA:SA Directory. Launch SA:MP, you'll see this button

. A text box will pop up and you'll have to use this IP Address: samp.argonathrpg.com:7777 and hit OK. Right Click on the server and hit connect and you should be in good shape.
If your game isn't launching, you can search for a GTA SA U.S 1.0 patch.
If you have any questions, feel free to ask