Q: i phone 6s wont sync with computer

Your Mac must be running v10.8.5 or later in order to sync an iPhone 6s >  http://www.apple.com/iphone-6/specs/

If your Mac is running  v10.5.8 as your profile suggests, it's doubtful you will  be able to upgrade to v10.8 Mountain Lion because your Mac must be running at the minimum v10.6.8 Snow Leopard in order to upgrade to v10.8 Mountain Lion >  OS X Mountain Lion system requirements - Apple Support

In short, you need a new Mac to sync your iPhone 6s.