Not trying to stop you from tackling this yourself, but, this can be a complex repair. You'll need a DMM, Scan Tool and a FSM to pinpoint the problem without replacing parts needlessly.
I'll give you some pointers anyway.
For P120 You need to test the circuits for the TP Sensor 1, TP sensor 1 and 2 are combined in one sensor, which is located on the throttle body. I don't think its serviced separate from the throttle body.
For P2120 and P2121 you need to test the circuits for APP Sensor 1. APP Sensor 1.2 and 3 are combined in one sensor and it is located on the accelerator pedal. The APP is not serviced separate, it's serviced as an accelerator pedal assembly.
I find it odd that you have both a TP and APP Sensor DTC's, my guess your problem is a wire or component that is shared to both sensors. In your case both the 5-volt reference circuit and the TAC Module are shared between the two sensors. I highly doubt that both sensors went bad at the same time.
Those codes should put you into reduced engine power, since the ECM or TAC Module does not know what position the throttle blade or the accelerator pedal is in. Reduced engine power or engine shut down is a safty net, it's far better than an unwanted acceleration situation.