Do you know how to edit the settings of your router?
It'll be in the wireless section of your router's settings. Maybe wireless security, look around.
Note: you need to be directly connected to the modem via ethernet cable or USB.
Don't know how? Fear not!
Some routers use the IP address 192.168.1.1. You need to type that into your web browser and when it asks use the password "admin" (no quotes) and leave the username blank.
If this ip doesn't work you can find out what you router's ip is by going to command prompt (Start>Run>CMD) and type "ipconfig" and find the Default Gateway, this is your router's IP.
Admin password not working? Fear not! Look underneath your router and sometimes they have the default username/password.
Default password not working? Fear not! You can set it back to default by holding down the reset button on the back of your router. *WARNING* This will reset ALL settings to default, meaning NO WEP code will be activated until you set it. (Fyi WEP sucks and can be cracked in 5 minutes, use WPA)
Also, you can find a manual online at google.com by typing in your router's model followed by "manual".
Ask if you're confused about anything.
Sorry about all the run-on sentences, I'm on my phone and too lazy to fix it all.