I gave up on MikroC a while ago but I can tell you the basic principle:
Connect the pulse circuit to T0CKI.
1. Set up TMR1 to generate an interrupt once per second.
2. Set up TMR0 to count pulses on T0CKI
3. At each TMR1 interrupt, read the value in TMR0, this is your pulses per second measurement.
4. Re-load TMR1 for the next one second period.
5. Clear TMR0 so it starts from zero again.
You might have to stretch the geiger pulses using an external timer. The pulses from the tube are extremely short and the PIC may not respond to them. You need a high impedance, high voltage to VDD conversion circuit anyway so you can easily incorporate it in the same hardware.
Brian.