"Sensor is ready"

What kind of output does it have?

Mostly it could be a analog output.

Process ( amplify ) the sensor output to get a reasonable amplitude pulse.

Have a Schimdt trigger to convert to sharp edged pulses.

Measure the period of the pulse.

Calculate the frequency and multiply by 60 to get the pulse rate..

And in case you are using some kind of sensor that has no galvanic isolation with the guy/gal whose heart you are monitoring then WATCH out. Safety comes first...then the pulse..

Raghu