Basic SVN tutorial:

FIRST. Download a SVN client. I suggest using tortoise which can be downloaded from: http://tortoisesvn.net/downloads (you will need to reboot after you install)

Now you will need to set up a folder for each SVN as well checkout into the directory. Here I will list the basic SVNs.

WIREMOD:

Will start with wiremod. I use the FULL wire SVN which will give you wire, wire model pack and ADV Duplicator.
Find your addons folder inside garrysmod. An example of the dir would be:
C:\Program Files\Steam\steamapps\havoc@clanvortex.com\garrysmod\garrysmod
Right click on your addons folder and click "SVN Checkout"
For "URL of Repository" put in this link: https://wiremod.svn.sourceforge.net/svnroot/wiremod/trunk
Make sure the "Checkout Directory" points to your addons folder. An example of the Checkout Directory would look something like this:
C:\Program Files\Steam\steamapps\havoc@clanvortex.com\garrysmod\garrysmod\addons
It will tell you the directory is not empty and are you sure you want to install, say yes. If it asks for a username and password.
Username = anonsvn
Password = anonsvn

PHX3

Go to your addons folder and inside your addons folder make a folder called: PHX3
Right click the folder for SVN check out and put in the URL: http://phoenix-storms.svn.sourceforge.net/svnroot/phoenix-storms

Other SVN links for gmod can be found on this website: http://wiki.garrysmod.com/?title=SVN_Links

You may also want to download Tactical Weapons pack all in 1 from garrysmod.org This will allow you to see custom weapons that are used on some servers. You can get that at:
http://www.garrysmod.org/downloads/?a=view&id=60980