Open Internet Explorer and type 192.168.2.1 into the URL address bar as though it was a website. You're then at the login page to the router's settings utility and admin will usually be the login with either admin or password as the password.
If you're in, seek out Wireless and then Security and take a note of the key. Sometimes it may be a passphrase, some a simple set of letters and others a hexadecimal code. You also may as well note which level of security it is - WEP, WPA or WPA/PSK 2 and then either AES or TKIP. Your computer will already know this but it's handy information to store. Note also that hexadecimal only contains numbers from zero to 9 and letters a to f so there's no confusion of I and O with 1 and 0.
Keep a note of that security key in My Documents and also outside the computer.