The bleeder valves are always near the top of the radiator, and usually on one of the ends. There are different types of valves, but they all work essentially the same.
For opening the valves, some have a knob to twist with your fingers. Others use a screw driver, but most need a square shaped bleeder key. If your valve has a small square metal pin in the middle, you need the key. These keys can be picked up at any hardware store for under a dollar.
TO BLEED THE RADIATOR:
NOTE: Please wait until after the heating system has cycled off for a period so the system's water has a chance to cool down before bleeding...Otherwise the water may scald you when it shoots out!
Unscrew the valve slightly until you hear the air escaping. Let the excess air out of the radiator until you get a good steady stream of water flow. Then let about a pint of water stream out to get all the bubbles out. Be sure to hold a pot under the valve as a lot of water can stream quickly out. Once the air is purged, screw the valve closed. Your radiator should heat up fully once the system cycles back on again.