If you're just driving it empty, I'd just leave it in high and not even screw around with it. You've got plenty of power. If you're hauling loads with it, then you can get acquainted with the two speed.
If you just want to play around with it, for starters go 1-2-3-4 in low, then when you're in fourth, shift it into high.
In fact, if you're not loaded, you should be able to start out in 2nd.
