You should have a radiators key. It's got a round barrel with a square hole in it. If you don't have one, get one at your local hardware store. Find the valve in your system that lets you add water to the heating system. It should be on a line that run from your cold water lines to your boiler. With the heat on and up to operating temp and pressure, open the valve and let some water in. You should see the pressure in your system rise. Take it up about 10 psi from normal. Don't let it get to high of you could create leaks in your pipes. Start at one radiator, hold a cup or bucket under the bleed valve and open it with your key. If water comes out close it. If it hisses or spits, leave it open until water comes out then close it. Do this to each radiator in your house. Check the pressure when you are done. Adjust it back to the normal operating range. If you had alot of air in your system, you will probably need to add more water. If you didn't find much air coming out, you may have to drain some water.