How to split voltage from a DC power supply?
You need a 12V to 5V DC-DC step-down converter (also look for: switching regulators). You can find plenty of them in the usual electronics parts vendors.

You could also consider a linear voltage regulator, but that would mean a maximum power dissipation in it of about \$ P_d = (12-5) \cdot 1.2 = 8.4W \$, which will (almost surely) either fry it or drive it into thermal shutdown. Also, it's very inefficient power-wise. Not recommended at all.