Visit www.garrysmod.org for addons.
A guide on installation is available at:
http://wiki.garrysmod.com/?title=Addons_For_Dummies
It depends on what you want to download, first get what you want to download off of garrysmod.org Maps go in the steam/steamapps/(username)/garrysmod/garrysmod/maps file addons go in the steam/steamapps/(username)/garrysmod/garrysmod/addons file
nowhere
you must buy it its 10$ for a downlod on steam
when you have downloaded an add on, extract the file to: :C/Program Files/steam/steamapps/<your user name>/garrysmod/garrysmod/addons. you put in maps to : :C/Program Files/steam/steamapps/<your user name>/garrysmod/garrysmod/maps.
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
Go to the garrysmod download site available in the related links.
Go to garrysmod.org and download various addons. These after download are usually extracted to the [Steam Install Folder]/steamapps/[Steam User Name]/garrysmod/garrysmod/addons folder. But these addons have installation instructions most of the time.
when you download an addon, put in in program files, steam, steamapps, <your user name>, garrysmod, garrysmod, addons. extract all files to this folder.
You put it in garrysmod\garrysmod\addons
Go to windows explorer (go to my computer). Browse to where you installed steam. For example "C:\Program Files\Steam" and from there click on the folder called steamapps, then your username, then click on the garrysmod folder, and inside that click on another garrysmod folder, this is where the garrysmod files are. For example: C:\Program Files\Steam\steamapps\<username>\garrysmod\garrysmod
I'm not sure which gamestartup file. But you can have background music when you start garrysmod, this file is gamestartup.mp3. The file can be placed here: <STEAM USERNAME>\garrysmod\garrysmod\sound\ui Hope this helps.
it is very simple. -download tortoise svn -make a new folder on the desktop called "wiremod"