There should be a Reset button buried inside a small hole on the back of the Router. Need to use a ball point pen or a paperclip to reach in and depress the Reset switch. This will reset the Router to original condition. Then go back in and change the user name and PW to something you can remember. That done, go into Wireless Security and set up the Encryption type and passphrase. Finally set up your wireless devices to match the encryption type and passphrase that you just configured on the router.
Before you do all of that, you might want to just check to see if the Router has already been Reset to factory and if Admin and Password will get you in.