First, look around and see if you have a router. A router is a small, usually blue, box connected to a internet modem. If you have a modem do this:
Go to the start menu and find the run button. When the prompt comes up type 'command' (without the 's), then click OK. Now type ipconfig and hit enter. Several lines of text come up. Look for the line that has 'Default Gateway' and copy what it says next to it into a new Internet Explorer (or, if you want, Fire Fox) window. Once the page loads look for a 'Virtual Servers' or 'Port Forwarding' link. Once the new page loads type 27016 (UDP) then press apply. Now you will need to restart your router by either unplugging it for 60 seconds or finding a small 'Reset' button usually found on the back of the router.
Also, you may need to disable your windows firewall / any other firewalls you have running at this time.
Starting the Server
Now that you have your computer set up to handle the listen server, start up Garry's Mod and click on the 'Create Server' button on the front menu. After the game has loaded you may need to type this in the console:
Now that you have your listen server set up and online you will need how to control it and it's players. The following are some good commands to remember. Remember, // means a comment.
// Kicks whoever's username you type in
addip <time> <ip>
// This adds the IP address you entered to the ban list for the time you gave in minutes.
// To find the IP address type 'status' in the console to find the user's IP.
// Displays all users on your server, they UserID, SteamID, and IP address.
// Either turns cheats on or off. Cheats enable ent_fire commandd, noclip, ect.
// Changes the number of allowed things. In this case, props.
// Props can be replaced with effects, spirites, ragdolls, ect.
// Either makes it so noclip is disabled or enabled while cheats are off.
// Gets your server on the server list instantly. It usually takes around 5 minutes.
If you are like most people, you can always just use a handy admin script to memorize all these scripts for you. Here are a few scripts that I recommend.
Ulysses Mod - An excellent mod that allows you to do anything from freeze players to stop players spawning props. This is the most powerful admin mod out there due to it's ability to give players a certain level of 'access'.
Hunt's Mod Revamped - A simpler admin-menu to use than ULX, but with much less features. This mod only allows the host (you) to admin your server with just a click of the mouse.