How do you i download things onto my garrys mod game

Depends on what kind of things you want on your game
for addons, and Maps get them off of GarrysMod.org

Heres how you install them

Program Files > Steam > SteamApps > YourUserName > GarrysMod > GarrysMod > Addons
Program Files > Steam > SteamApps > YourUserName > GarrysMod > GarrysMod > Maps