From SA-MP Wiki
The first step is to download the SA-MP client. The download page can be found at http://www.sa-mp.com/download. Check back there from time to time for new SA-MP versions, especially if you're getting "Incorrect Version" errors.
After downloading the latest SA-MP client, run the downloaded .exe file and follow the instructions.
The installer will ask you to locate your GTA:SA installation path. By default this is 'C:\Program Files\Rockstar Games\GTA San Andreas', however yours may differ.
It is very important that SA-MP is installed in the same place as gta_sa.exe. This is important as SA-MP needs the core San Andreas files to run. You should see the following window with your San Andreas directory pre-selected. If it is selected, click install. If not, browse for the directory.
This automatic installation process should place a shortcut named SAMP on your desktop, if it's not there check your GTA San Andreas directory. Run this file to open the SA-MP server browser, where you can join a server to play on.
Finding a Server
Finding a server is easy; simply run the newly-created shortcut on your desktop (samp.exe), and the server browser will appear. Click on the 'Internet' tab at the bottom of the client to show all servers that are publicly visible. The 'Hosted' tab is a special list of servers who have paid to appear on it to gain visibility.
The only necessary user input in to the server browser is to enter your nickname in the 'name' field to the top of the browser window. Then, simply choose a server you want, right-click on it, and press 'Connect'. Alternatively, click the green 'Play' icon in the top right corner. If all goes well, this will load up the game and connect you to the selected server. You will then be playing San Andreas with anywhere up to 1000 other people!
Your First View
The first you will probably see in any server is the class selection screen, where you choose the class/skin you wish to play as. In some gamemodes choosing a skin will choose what team you are in, or what role you will play in the game. Different skins may also have different weapons; it is all server-specific. In some gamemodes, classes/skins do not have any special abilities, weapons or roles - it is simply a cosmetic choice. To scroll through available skins press the left or right keys on your keyboard, and press your Shift key to spawn. Alternatively, use the mouse cursor and GUI buttons at the bottom of the screen. If a skin is part of a certain team then scrolling through them will generally show which team you will be on if you were to choose said skin.
The chat box/window can be found in the top-left corner of the screen, and provides a way for users to talk with each other, send commands and receive information from the server. To open the chat input box, press T, F6 or `. You can toggle the chat window using the F7 key.
The 'death window' can be found in the middle of the right side of the screen. The list shows the 5 most recent deaths, showing the killer, victim, and the weapon used. It also shows suicides and generic deaths such as drowning or falling. It can be toggled by pressing F9.
As SA-MP is built on top of the San Andreas game, almost all the keys are exactly the same as single player with the exception of a few SA-MP-specific commands.
|F1||Displays the in-game help menu|
|T||Opens the chat input box to type chat and send commands.|
|Shift||Select a class while at the class selection screen (or press the 'SPAWN' button)|
|F4||Allows you to select a new class upon your next death.|
|F5||Shows debugging information (e.g. packet loss and memory usage)|
|F6||Also opens chat box like 'T'. Another alternative is the ` (grave) key.|
|F7||Toggles the chat window. Pressing once will remove the chat outline, and pressing it twice will hide it and the rest of the HUD.|
|F8||Takes a screenshot. Screenshots should be saved in My Documents in the 'GTA San Andreas User Files\SAMP\' folder.|
|F9||Toggle the death window.|
|F10||Hides the chat and HUD while held.|
|G||Press near a vehicle to enter as a passenger.|
|H||Press while a passenger in a suitable vehicle with a suitable weapon to perform a drive-by.|
|Esc||Cancels current action/selection, including chat input, dialogs, textdraw/object etc. selection and more. If there is no action/selection to cancel, it will open the GTA:SA pause menu.|
As for all other commands/keys, they are exactly the same as single player. They are mapped in the pause menu settings. If you have never played San Andreas before it is highly recommended that you play through Single Player first, even if it is only the first few missions. A lot of playing SA-MP involves knowledge of single player itself, such as where things are (e.g. weapon shops). Most servers will likely have some form of 'helpful hints' for new players though.
Enjoy playing SA-MP!