Assuming that you didn't get a manual with the unit if you bought it 2nd hand................
Press and hold down the MENU key. The route menu appears on the screen.
There's a button marked set HOME waypoint.
press this button.
Choice is now "Current Location, Use Map and Find.
Click on Current Location (when you are at home, doh!)
Touch "OK"
Screen changes to New Waypoint screen. Click OK, and home is set.
To plot a course from any location back to your home, just press the FIND button twice. A choice is shown if you want to get home. press YES and away you go.
If you did get a manual... well then