I'm looking to use the I/o of an at tiny to power a transistor to turn on a 12v 1.8amp circuit. Input to the tiny is 5v. I'm thinking I need a second transistor? Help sizing the transistors would be much appreciated.
You need a regulator to reduce the 12v to 5v. You could use a linear regulator like 7805 but it would be more efficient to use a switch mode regulator.
You could also consider using a logic level MOSFET to drive your output.