open your web browser, type 192.168.1.1 the default username is admin and the default password is admin, if that doesn't work someone prolly changed it, and you should press and hold the reset button on your router for a few seconds, then try it again. then when you login, goto Wireless, then goto Wireless Security and enable "WPA Pre-Shared-Key" and type in a really good long random password/key, also, sometimes people will enable remote administration on the router so they can take encryption off if someone puts it on, goto "Administration" to change the password, and disable remote management.
but before you do anything you should reset the router because you don't know what settings people have changed.