I don't know how to stop

There are various ways to stop on a skateboard, such as dragging the tail or doing a powerslide, but dragging the sole of your foot on the ground is the easiest method. Keep your front foot on the board as you would when pushing, and once you can support your weight with just your front foot, gradually lower your back foot to the ground and slide it along the surface. If you suddenly place your entire foot down, you are more likely to lose balance, so the key is to lower it gradually.