To port forward you will first need to first look up your IP. To do this you can go on google and simply type "What is my ip" and it should pop up in bold letters. Copy and paste that into the address bar and press enter.
Username is usually : Admin1, or Admin
Password is usually: password
After that it is going to prompt you to change your password, please change it to something memorable and i suggest writing it down, because getting your pass back from your service provider is a pain in the ass.
Once you have done that you are going to want to find and click on "Port forwarding" and if that's not there try finding "Firewall settings"
If you have "Firewall settings" and not "port forwarding" please follow this next step.
You are going to find port forwarding or gaming ports right after clicking on "firewall settings"
Once you are in your port forwarding section of your router, you are going to then want to click on "new port", or something similar to that. I have a verizon router so for me it will say "Create new port forwarding rule". Click on the down arrow and find the ip with your computers name beside it. Then you are going to want to choose "application to forward" or something to that extent, and click on "custom port" for where it says "TCP" click on the down arrow and choose "both" or if that's not available and it only says "TCP" and "UDP" you are going to have to port forward twice.One for UDP and one for TCP. For where it shows a blank box, type in "25565".without the quotations. You will then click "apply" or "ok" or something like that.
Then scroll down to the bottom of your list of ports, you will find the one (or two) you just added and check the boxes right next to them. Then at the bottom of the list, click on "apply changes" and you should be good to go
This also works for other games as well that don't forward for you, but you have to type in different numbers then "25565" you would have to search up their ports to find out.