I am using a wheel, a PIC, an accelerometer and 2 DC Motor. 1 DC motor is needed to move forward while another is need to move backwards.
"A wheel" like "a single wheel"??
Which PIC? (there's several hundred different PICs)
What do you want the accelerometer to do?
Why not use the same motor for both directions??
From the above, do i need to generate and control the PWM of the chip to drive the 2 motors...?
From the above, you need both cunning and luck and plenty of both to succeed.
I get the impression that you are in deep water here, so back up a bit and tell us what the assignment (I suppose it is) asks you to do.
and if i'm using accelerometer, do i need to write a programme for analogue to digital conversion...?
Yes, if it's an analog output sensor.
Perhaps you should take a look at a very simple robot like the "$50 Robot" from this site (AVR based) or a similar contraption from somewhere else (Google) to get the feel for it.
When you have a handle on the basic stuff, you can add an accelerometer.