How to find feature haar like image, rectange containg the region to car recognition

i do project about car recognition and counting number car in image. the first,I connect webcame take a photo and save in folder (input data) then i take more a photo to compare with input data but i donot know how to compare two image and recognition i read some document find feature haar like two image then compare them but i donot understand Please, help me? thanks u so much