Go on your computer
Open up any browser
Type in your IP Address of your router which is usually 192.168.1.1
It will prompt for a password and username
If you have never changed these then it will be
Username: admin
Password: password
Check under "Wireless Settings" and scroll down to your password.
I highly suggest you do not use WEP mainly because they are very easy to hack. Instead, change to WPA2 and create a new password.