FSR readings

How can i convert FSR ( force sensitive ressistor with 2 pins) analog readings into newton ?

Identify the force sensitive resistor you have been asked to use, locate it's datasheet, workout how an analogRead() could be used to determine the force applied to the sensor and, if necessary, google for a conversion between the sensor output units and Newtons.

6v6gt:
Identify the force sensitive resistor you have been asked to use, locate it's datasheet, workout how an analogRead() could be used to determine the force applied to the sensor and, if necessary, google for a conversion between the sensor output units and Newtons.

Or, apply different forces in Newtons to whatever FSR you have. Write down the force in Newtons and the raw analog value in a table.

void setup() {Serial.begin(115200);}
void loop(){Serial.println(analogRead(A0));delay(1000)}

Now that you have a table of raw analog reading mapped to Newtons of force you can use MultiMap to do a piecewise linear interpolation or use some kind of polynomial curve fitting software to calculate a formula.
https://playground.arduino.cc/Main/MultiMap/