Given the HP and torque of the 5.0, the GT should be a fairly easy car to learn to shift with a manual.
I would think that the first thing you should do is have someone with you when you pick up the car who knows how to drive a 6 speed really well.
Have that person take you to an empty parking lot where you can take over and practice. Watch the tachometer, and as you take off from a stop, use no more than 1,500 rpm. Match the clutch take up with additional throttle to keep the rpm's at 1,500. This will take a few tries, but once mastered, you'll be able to do this without even looking at the tach.
As you learn to do additional gears and on up to speed, don't go crazy with rpm and speed until you gain experience. 435 HP is nothing to fool around with for a novice.