i want to make a counter which counts white strip on flywheel and display the count on serial monitor...can anybody help me out?
Do you want to learn how to do this or have someone do it for you? What programming and electronics background do you have? Do you mean serial monitor or something like a LCD display? .
I believe this could be the sort of thing you are looking for:
Basically you are looking for a Tachometer. For this you could also use whats called a Reflective Photo Interrupter and use a standard Photo Interrupter Tutorial as follows:
For the Photo Interrupter route you would need to adjust the resistances and tolerances in the code accordingly so it can detect the difference between the white line and the colour of the flywheel. Shouldn't be too hard and with some playing around you should be able to get it set just right.