The main goal of the game is for you to complete all levels and collect all data in the game to earn credits that can buy new weapons and other items for you to progress further into the game.
If you are looking for tips about controlling the characters in the game or the weapons used, you can find them over at the San Andreas: Multiplayer in-game help menu. There you will see the in-game menu with various game modes, you can select any of them that you prefer to play. The first thing you need to do is to launch San Andreas: Multiplayer, then go to the main menu and select Multiplayer. You can add new maps and game modes to your game through these files. These are necessary files so that your game can run as smoothly as possible. There are many files and folders in your computer when you install San Andreas: Multiplayer. It also has a list of maps and types available for you to choose from. This zip contains further info about the configuration of your server using a multiplayer mode.
The official website of San Andreas: Multiplayer does not own nor develops this game.