As suggested, to make the car go forward you could just use a battery and motors.
As for the 'forward' aspect: the most obvious way to make a car is to use two geared motors to driver a left and right wheel, but this will result in a car that moves approximately forwared, plus or minus maybe 90 degrees :) Powering the two wheels from one motor and using a servo motor to steer the other (front?) wheels is more liklely to result in a straight line movement.
As for the PIC, please consider a less stone-age one. Just a few suggestions: 16F886, 18F2520. Both are MUCH more powerfull, 28 pin, and cost less than a 16F84. (Does anyone still sell those chips??)
From your question I think you should consider something simpler than a uC-controller car, maybe google for 'beam robot'? The term beam is used for robotic vehicle like thingies that use very very simple hardware, and often solar panel power.