Here is just a collection of various tutorials and various How-To's for various subjects.

Download and Play GTA San Andreas: Multiplayer!

Go to "http://www.sa-mp.com/download.php" and download the program in the "Client" section from any of the websites given. Install to any directory and run the file called "samp.exe" in the file you installed to. You will now see the main window called "San Andreas Multiplayer v0.1b". You are currently in the "Favourites" section, so click on the "Internet" tab at the bottom of the window, and choose a server. Press "Connect" and play! Quite addictive. Oh, near the top of the window, is a textbox where you can create/change your nickname.

Quick Controls:

g - get into car as a passenger

t - chat to other players in the server 

Create your own Half-Life2 map!

 I won't bother writing a tutorial for you because there is already an excellent one provided by Valve. Here's the link: "http://developer.valvesoftware.com/wiki/SDK_Docs".