To get started studying up on the Garmin GTN 650 we suggest the following:
Garmin Learning Center, Video Introductions
You will see that the Garmin GTN 750 and GTN 650 are used in the demonstrations, the units are quite close to each other and the differences can be found quite quickly.
If you own an iPAD we recommend the GTN 650 / GTN 750 iPAD app. You will actually use the unit by touching the screen which is how the GPS works to begin with. Available in the iTunes store.
GTN 650 / GTN 750 trainer application for PC:
This is available from Garmin and is an excellent help to study up the functions and the workflow of these boxes.