How do i make a password on my wifi conection..my router is linksys BEFW11S4 v.2

First, your router is old, go pickup a new one that supports the G standard, don't worry it will fall back to B standard or better yet get an N standard router. To setup wireless security directly connect a pc via cat5 data cable to one of the switch ports on the router (not the wan) and login to the web innerface, click the wireless tab, now look for the security or encrypted options. Most routers support WPA-2 encryption where you enter a phrase of 2 or 3 words if your router only supports web encryption go buy a new router. Save it, then go to your pc, disconnect, remove the wifi ssid from your managed wireless list refresh and search for the ssid of your wireless signal (name), now connect and the router should challenge you for your WPA pass phrase you setup in the router. Now your wifi ssid is secure and all connections will require the passphrase.