What is SourceMod?
You can look at SourceMod as the all in one admin solution for Source based gaming servers (HL2 mods etc).
Its like AMXModX was for HL1 but then better. Basically the only thing you need in order to run SourceMod is a valid Source based gaming server and MetaMod: Source installed.

How to install MetaMod?
MetaMod is an API handler for the Source engine that takes care of important tasks, in order for SourceMod (that runs under MetaMod: Source) to work you need to install MetaMod: Source first. To do this follow the steps below:

1. Download the appropriate package for your server: there is a Linux tarball and Windows zip file. You can get this from the MetaMod: Source Website.
2. Open the package, you will either have “server_i486.so” or “server.dll”.
3. Find your “mod folder” on your server. This will be something like “cstrike” or “hl2mp”.
4. Make a folder in your mod folder called “addons”, if it doesn’t already exist. Make a folder inside addons called “metamod”, and a folder inside “metamod” called “bin”.
5. Create a new, blank, text file called “metaplugins.ini”.
6. Upload the files to your server:
   • Upload the server.dll or server_i486.so file to your addons/metamod/bin Folder.
   • Upload the metaplugins.ini file to your addons/metamod folder.
   • WARNING! – Do not replace the mod’s server.dll/server_i486.so (usually in “<mod>\bin”). This will cause your mod to stop working.
7. Open the file in the mod folder called “gameinfo.txt”. You will see a few lines at the bottom like this:

   SearchPaths
   {
   	Game				|gameinfo_path|. 
   	Game				cstrike
   	Game				hl2
   }
   

8. Add a line after the “{” sign but before all of the “Game” entries that looks like this:

   GameBin				|gameinfo_path|addons/metamod/bin
   

9. You’re done! To test whether it worked, restart your game server and type “meta” in the server console. You should get a menu.
10. Optional Cvars: mm_pluginsfile – Sets the path and name of the plugins file, defaulting to “addons/metamod/metaplugins.ini”. It is relative to the mod directory.

Installing SourceMod
SourceMod will run on any mod built using the Source SDK. It also supports “The Ship,” which uses the Source engine.