You need a file extractor to download most mods. I recommend 7zip or Winrar.

Open up your file explorer and go to Local Disk C and open up the file, "Program Files (x86)" now opening x86 is very important so double check if you got the right one.

The best way to find mods for HL2 and other source games is Moddb, look up on any browser Moddb Half life 2 (or source game of desire) and find mods you want. Mods I recommend? HL2 MMOD and The Stanley Parable. Now press the download button you'll see at the image you see. Now Press the latest version of the mod you want.

Extract the mod you got into a normal folder now open a new window, remember Program File (x86) now find in there Steam, while all of the files there may be scary . Find "steamapps" no spaces and open it, then open "sourcemods" then drag and drop the mod you got into the sourcemods folder than BAM ! You're just one step away from being done Now after that restart Steam click the ^ icon on the dashbar on windows then right click Steam then press exit steam.

Thanks for reading, I really hope this helps someone out there trying to install mods for the first time!