it depends who your router is made by. most common ips are 192.168.1.254, 192.168.0.1, 192.168.1.1. try entering each of them into your address bar. if they don't work you need to find out your default gateway, just google your router model and put default gateway after it.
once you're in the settings don't change anything you're not sure about. all routers are different but you'll proly need to go into wireless settings and change it somewhere there, or advanced settings then wireless settings.