Q: My iPhone 6 wont connect to my iTunes.

Are you running iTunes 7.5.1 or newer?  https://support.apple.com/HT201352

What about your computer OSX version?

iOS10 requires iTunes 12.5.1 or newer which in turn requires OSX 10.9.5 or newer.  Software Update only checks for updates for the current system version you are running but that itself may be out of date.  It may or may not be possible to upgrade your computer to the required system. Find your computer model on the http://www.everymac.com web site and near the bottom of the system specification section it will say what OS versions it is capable of running. If you cannot run a newer system you will not be able to sync this phone to your present computer. OSX 10.9 and 10.10 are no longer available. Check this article on upgrade options: If your Mac is not Compatible with Sierra but you Have Not Upgraded to El Capitan  - https://discussions.apple.com/docs/DOC-10692  El Capitan (OSX 10.11) and Sierra (OSX 10.12) may run slower on older machines and require additional RAM purchase (4 GB is a realistic minimum and many recommend 8GB).  Doing a big jump in system versions is also more likely to affect old software.

Snow leopard to el capitan, will it cause my macbook to be slow?  - https://discussions.apple.com/thread/7412959

Current OSX general upgrade information, including system requirements - http://www.apple.com/osx/how-to-upgrade/

If you have any PPC generation software that has been running under OSX 10.6 it will not be supported under any later system versions.

You may be able to update to the required iTunes version after you have upgraded to a required OSX version. You tell us nothing about your computer other than it is an iMac (which covers about 20 years of computers) which is why I provide the self-help information in my earlier post.  My MacBook came with OSX 10.5 and is currently running OSX 10.9.5.