help with placing diodes and upgrading 5v servo to 12v servo?

how to program in UART to control a digital servo with an arduino?

That is just Serial.print and Serial.read

where do i place the fuse in the circuit?

In series with the positive lead to the battery.

Watts = volts x amps. What am I missing?

That a servo has electronics in it and even if you could supply the motor with more voltage independently it would then get too hot and burn out.