Firstly you need to access the admin page, preferably while all the pcs are connected to it by ethernet.
Try typing the following in to the address bar:
http://192.168.1.1/
Then go through the points in this:
http://www.practicallynetworked.com/support/wi reless_secure.htm
The minimum is to change the admin page name and password, the SSID name and disable broadcast, and set WEP, but WPA is better.
If you do it without being connected by cable, you may block your own access!
Find the reset button on the router so if it goes pearshaped you can reset the default settings.
Make a note of all your passwords and settings.
When you are familiar with the admin page you can set your MAC filters.