Tire Iron BeadBrakR slide show - BestRest
This shows the valve core removal tool.  Place this over the end of the valve stem and twist it slightly until it engages the valve core.  Then unscrew the valve core.  

Note: if there's any air in the tire when you're removing the core, that air pressure will cause the valve core to blow out of the end of the stem when you reach the ends of the threads.  Be ready to catch it in the palm of your hand

This shows the valve core removal tool. Place this over the end of the valve stem and twist it slightly until it engages the valve core. Then unscrew the valve core.

Note: if there's any air in the tire when you're removing the core, that air pressure will cause the valve core to blow out of the end of the stem when you reach the ends of the threads. Be ready to catch it in the palm of your hand