As usual, I would suggest using a more modern device, rather than the long obselete 16F84?, but if you've already got one - fair enough.
All you need to provide the PIC is a squarewave pulse from the sensor, then simple measure it's frequency - or, more likely, it's period - the time between two pulses. From this it's simple to calculate the speed - assuming you know the relationship between the pulses and speed?.
You can do this is many ways, you might check the really old MicroChip application for a frequency counter, or the magazine EPE have done a number of projects like this (check PIC Agoras, which is a speedometer for any wheeled device).
But essentially, wait for the a start of a pulse, then start counting - while counting check for the start of the next pulse - when you find the start of the next pulse stop counting. The counter now contains a value representing the period of the incoming frequency - simple maths can convert that to frequency or speed.