I've installed Drupal Commerce module, I got show cart block in header. If I click on it takes to cart page. I need a pop up of cart page on clicking show cart.
Try the Commerce Ajax Add to Cart module.
Using this module you can ajaxify the add to cart operation. The updated cart will be displayed without page refresh and a popup message will be shown after you add an item to cart. You can customize the look of the blocks using its template files.
Along with Krishna Mohan suggestion you can try Commerce Ajax Cart.
This module adds a pop up cart that is AJAX handled.