What router? Different routers are access in different ways and at different addresses. Models from Apple use the Airport Utility program. Most other routers are accessed through your browser at some specific IP address. Typical router addresses include:
There are MANY other possibilities, especially if hte router's config has been modifed from its default setting. Consult your router's manual for the default address.
Another way to get the address is to do an "ipconfig /all" on your Windows computer and look for the Default Gateway setting on your active connection. Other OS'es similar methods, just find the default gateway setting in your TCP/IP settings on your active connection.