what do i have to do to put a 2004 3.8 engine into a 1995 car

You will most likely need to change the wiring harness and some of the sensors. Rule of Thumb: If the sensor is different than the one in your car either verify that it sends the same signal bake to the ECM or just swap it with the original from your car. You'll have to use common sense with this as most people here would not know the differences between each component on that car. When I do my engine swaps I always use the units from my car in the new motor because I know they work. Then test the ones that came with motor, keep the good compatible ones.