So, if your new to Garry's mod, your most likely going to want to know how to install addons. In this small tutorial, i will show you how.
1: Download an addon. (you cant do this is you pirated the game) It doesn't matter what addon, but one you like. (or multiple) you need winrar to extract the addons.
2: Types of addons. There are lots of types. One of the most common are maps. They are levels you can play in. There are the all famous, Lua addons, witch range from weapons to head bobbing like in Mirrors Edge. There is also skins for gmod, witch you can infact find right here on the site. But for gmod stuff, i suggest garrysmod.org -its all gmod based. (well some people upload far cry stuff but i suggest that you dont) There's also backgrounds, witch is a picture that appears when you start the game. (CAUTION!: GMOD.ORG HAS LOTS OF PORN BACKGROUNDS AND CRAPPY BACKGROUNDS!) there's Tag boxes at the top to help you navigate. oh yeah, there's also adv-dupes, witch are contraptions you can spawn anywhere. (Requires you have the advdupelicator addon)
3: installing. Select a addon or map. open it to see whats inside. now, most addons tell you where to put it, some dont. So, if it says the addons name and its an addon, put it in the addons folder. If its a map and just contains 1 file (bsp) then drag it into the maps folder. (Path to these is: c:/program files/steam/Steamapps/username/garrysmod/garrysmod.) however, some map rars when you open it, will have multiple folders. for example: Settings, Maps, Materials, Models. When you see this, its kinda an indicator that you got a good map (but just bsp file maps are good too.)
Simpily drag it into the main gmod folder, the one containing all the folders and stuff. (LOTS of folders) some addons also have this, repeat the same thing.
4: other notes. If an addon doesn't work, Try opening the rar and the addons folder untill you see folders like lua or materials, then drag into main gmod folder. To uninstall addons, take it out of your addons folder, if you dragged it into your gmod folder then backup your addons/maps/saves/dupes and delete (or rename gmod folder like gmod old or LAWL) all contents of gmod folder. When you start gmod, it will replace the files. Make sure you ALWAYS ALWAYS read the description. as you might need games like tf2 or ep2 or css to use it. for some addons, you cant download from gmod.org. for the list below, addons that say svn beside them are svn. that means its THE most up-to-date version. you need tortoise.svn to get these addons.
5: HIGHLY SUGGESTED ADDONS. you almost NEED these addons.
Spacebuild Modelpack svn
Everything that gmod.org user sakarias has uploaded. (seriosly)
Css realistic weapons or madcows weapons. (need css)
Tf2 buildibles and sweps (needs tf2)
Some sort of vehicle pack
Dans snpcs final
Gcombat extras svn
Generics default combat svn
I hope this helped you. +)