There are two different passwords at issue here. They aren't necessarily the same, and shouldn't be the same.
One is the password you use to get into your router settings. If you don't know what that password is, you'll have to reset your router to its factory default settings. I can't tell you how to do that. There's probably either a button to press or a pinhole into which you insert a paper clip. After you reset the router, it will have a default password, which you must change immediately. The default password is in the documentation somewhere. Find that documentation on the manufacturer's website.
Do not leave the default password set.
The other password is the network password, which you set within the router's web interface. I can't tell you how to do that either. Make sure you select WPA2 Personal as the security standard, not WEP. If the router doesn't have that setting, recycle it and get a newer one.
See this support article for the possible formats of wireless network passwords:
AirPort: Joining an encrypted WEP or WPA Wi-Fi network