Recently starting myself so I can understand your frustration.
Sourcemod uses a scripting language called sourcepawn,
http://wiki.alliedmods.net/Introduction_to_SourcePawn
Very similar to C++ and basically is an extended shell language (like say a unix shell (bash) or games with shell engines like Morrowind.
The link above is your basic primer on variables and functions and is a good read.
Second, get some approved plugins and download them to your l4d/addons/sourcemod/scripting directory as a *.so file for practice (download the source not the plugin). Open the *.so file in textpad and have a look at the structure of the code. See how the modder achieves their goal, whether it be using netprop memory addresses, l4d convars, or whatever. Double click the compile exe in the same directory. This will compile your plugin and put it in the "compiled" directory.
It's hard to initially get your head around, but once you know where to find stuff its not that bad.