Inexpensive method for highlighting a sketch I am from India and thinking of starting sketching.

As a beginner I am going to buy 2H, 2B and 4B (I hope that is enough).

What I need to know is some inexpensive method/white pencil for highlighting.

Can I use the hard white chalk used for writing on slate?
I wonder what you mean by highlighting a sketch? If you work on a white (or almost white)  paper then the only thing you gonna need is a soft (6b) pencil or even cheaper a regular HB penci to start. But I recommend a 6B.
If you gonna work on a toned paper or cardboard then be aware that chalk won't work well on graphite. Also a black and a white one will give you a slight blueish cast if mixed. Thereby going with a 6b and leaving the highlights untouched is better. If needed you can use an eraser as a cheating option.