Stand the can right way up on a solid, flat surface. Make sure you have a pair of thick soled shoes on to prevent injury to your foot. Stomp on the top of the can to crush it.
Use a manual can crusher. These are available from hardware and home stores. Simply place the can on the platform and pull the handle down to crush the can.
Buy an automatic can crusher for large volumes of cans. Set the crusher up over a bin so that you can collect the crushed cans. Turn it on. Place the can into the slot and it will drop down and get crushed by the automatic crusher. You can place several cans into the slot at the same time to expedite the process.