If you have a default wireless connection activated, connect to it on your computer, or connect to the router using a cable.
Open up your browser and instead of a website address visit 192.168.0.1 or 192.168.1.1
Default username and password is "admin" I believe.
Click the "wireless" option and set it up.
If you need further information you should look up an user manual for your router, it will be easier for you.