More info is always helpful.
Also posting in the right area is a good idea, this is the Mac Pro area not the MacBook Pro area.
But I will assume that the Sony Trinitron is an older CRT monitor with a VGA only connection.
You will need to get a DVI-A to VGA adapter and connect that to the DVI port on the MacBook Pro, then connect that to the external monitor.
According to Apple the MacBook Pro is shipped with the proper adapter cable, if you lost yours then you can buy a new Apple one for $19.00 at
http://store.apple.com/us/product/M8754G/A?fnode=MTY1NDA3Ng&mco=MTUzNTg
or a cheaper one for $1.94 at
http://www.monoprice.com/products/product.asp?cid=104&cp_id=10419&cs_id=1041903&pid=2396&seq=1&format=2#description
Then open System Preferences and go to Displays and Detect Displays to activate the other monitor.