Very good question. I take it you do not want to understand how Source works with mods so I will be quick. First, check the readme and follow directions. If their are no directions you most likely have to take the file, if the file is large I would recommend saving it to your computer first, copy it, or possibly extract it to this path: Go into My Computer, double click the hard drive (that thing that should say "Local Disk (C
"), double click "Program Files", and then "Steam", "Steamapps", "Your user name for Steam", "Source Mod(s)". I'm not sure if you have to go to your user name, if after going into Steamapps you see Source Mod, go into there. Paste or Extract the folder into that file. Restart Steam if it is running and then you should have the mod now. If you do not, you may have to overwrite certain HL2 files and alike. Most mods REQUIRE HL2, and Garry's Mod requires HL2, CS:S, AND HL2: Deathmatch so make sure you have at least HL2 unless you are getting Garry's Mod. What mod are you trying to play?
If none of that made sense:
1. Copy Mod Folder
2. Go into My Computer and Double Click "Local Disk (C
3. Double Click "Program Files"
4. Now double click "Steam"
5. Then "Steamapps"
6. And if you see the folder now double click "Source Mod(s)"
7. If you did not find Source Mods yet, double click your username folder in "Steamapps" and then you should see "Sourcemods"
Once again, if this does not work, tell us what games you have on Steam and what mod you are trying to use.