How to transfer files from one iMac 2008 to a iMac 2015

If you're upgrading from an iMac 2008 to an iMac 2015 and wish to migrate your documents and files and related contents, click on the following blue-text link > Move your content to a new Mac - Apple Support

In what context?

How many?

Regularly or just once?

You can do everything from setting up a small network over LAN to just dragging them to a flash drive and then off again. If you are doing this as part of setting up a new computer there's special ways to do that. So we really need more context.