Go Start then Run in XP - Windows key plus R in Vista and Windows 7, then type in the following, including all the spaces:-
cmd /k ipconfig /all
then press Enter and from the black form that shows up, take a note of the numeric code beside the Default Gateway icon - something like 192.168.0.1.
Type exit to close the black form.
Open Internet Explorer and enter that code 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 to set up wireless security info.
Keep a note of that security key in My Documents and also outside the computer so you don't have to go through this again and I don't have to write it all out.