An IP address is a set of numbers, like 192.168.1.1. If you have a wireless/wired router, the easiest thing to do is to use a web browser and connect to the router. You can do this by entering an address of the router, as follows
http:// 192.168.1.1
(There would be no space between the // and the 192, but it's there to prevent this from becoming an internet address.) The address could also be 192.168.0.1, or any of several others.
Anyway, when you do this, you would get a login screen allowing you to access the router. If you know your router name, you can download a manual from the manufacturer's website. There is typically a way to assign each device on your network an assigned address (and you don't even have to care what the address is).
You don't have to do this, but it tends to make the systems more stable if you do. For instance, my wife's computer kept losing our networked printer, so I assigned the printer a fixed address, and her computer hasn't lost the printer yet.
