Using a Force Sensitive Resistor (FSR) as a switch
There is a nice tutorial about using an FSR with ADC of arduino:

https://learn.adafruit.com/force-sensitive-resistor-fsr/using-an-fsr

Here you need to modify the code so that when the ADC reading is above half (ie 1024/2=512 or at voltage reading of 2.5V), then you turn output pin high. When it’s below this threshold, then output of pin is low. 

I recommend using the example code(s) from link above with example code of using button with arduino and example code of turning transistor and motor on with arduino which you can find on google. 

https://www.arduino.cc/en/tutorial/button

https://www.arduino.cc/en/Tutorial/TransistorMotorControl