It should be in the documentation that came with your router. If you don't have it, try looking it up on the Internet. It's specific to the router, but it's common for most routers from a company to have the same defaults. Most can be accessed at 192.168.0.1 or 192.168.1.1, but they are free to pick from numerous (millions of) IP addresses.
If you still need help, people will need to know the exact model of router you have.