you must tell to TIMER0 where to take it's clock to increment and enable interrupts.

let's say you have a 16F628. you'll find the setting in the INTCON and OPTION registers

OPTION register
by default TIMER0 get it's clock source from pin RA4, on a high to low transistion with a rate of 1:256... is this what you want?

INTCON register
To make your interrupt routine work, you must, at least, enable the TIMER0 interrupt and the global interrupt... they're disable when you boot the PIC

So INTCON=%10100000


what about now?