You may be confusing one of several layers of password/user name levels.
1) Ignoring wireless for the moment, you need a username (usually admin) and password (often password or admin) to access the router setup screens. See documents which came with the router or download manual from maker's site.
2) Once you're in the router setup screens, WAN section, you'll probably see that a user name and password have already been set up if the router was supplied by your internet provider. If not you need to consult the paperwork which came when your account was set up and apply the name and password.
3) While you're in the router setup screens go to Wireless Security and see if a wireless security passphrase has been applied and which security system has been selected (usually WPA TKIP PSK).
If you are going to use wireless, either note down details or set a new passphrase.
If you have a wireless adapter in your computer apply the security type and the passphrase either to the wireless adapter's utilities or to Windows Wireless Connection.