How to port foward(Garry's Mod)

Warning: I'm no longer updating the guide or helping. I apologize
Due to YouTube being stupid. I had to make a new Channel.. youtube.com/user/P0isonMaid
You need a router.
This tutorial shows you how to set up port forwarding. In this case I am setting it up for Steam's Garry's Mod. I am using AT&T U-Verse.
If you have any questions about this, have problems, or need me to help you portforward on another service; please do not hesitate to PM me, add me on steam, or comment below. My steam name is [ち:き] Angel

-Ports used : 27015 to 27016

to find Device Access Code. Go to your modem. Find where it says
Device Access Code : _______
The 10 digit code will be the password.(Note : It may not be 10 digits)
Turning Firewall off will solve most issues.
Important: I forgot to add most times if you have connection failed after 4 attempts or whatever.. either the portforwarding is not correct, your ping for other people is too high, or you need to turn your firewall off. Also try right clicking and running it as admin. Mess with your port forwarding settings, the tcp, udp, or if you have both that option too. Sometimes you may need to use 27005 to 27006 numbers. Just make sure you try the different options you have to make it work for YOU.