Downloading addons ( Sweps, Maps, Game modes, etc. )

So i havent been playing Gmod for quite a while now, decided to start playing again.

I wanted to start downloading addons, but i remember in the past it never worked for me.
I am running Vista for my operating system if that makes a difference.
What i really need to know, is how to download addons, what location to save them to.
i think it was something like

C:\Program Files (x86)\Steam\steamapps(my username)\garrysmod\garrysmod

or is it

C:\Program Files (x86)\Steam\steamapps(my username)\garrysmod\garrysmod\addons

Yes, second one is where to install the addon, to tell if its an addon is to go into its main folder and see if it has a info.txt in it.