Remember to turn the heating off first!!!!
You must be made aware whether you have an open vented system or a sealed system as this will determine whether you need to add water to the system. When you bleed an open vented system the water in the system is automatically topped up by the feed, whereas in a sealed system you need to manually add water to top up the system. You would need to refer to your instruction manual on how to top up the system or consult a plumber for advice.
Bleeding A Radiator
Ideally you should use a radiator key as trying to use other tools of objects could damage the valve. you can pick up a radiator key from your local DIY store or ask your local plumber.
Use the radiator key to slacken the valve which will then release the excess air. Make sure you have a rag handy to catch any water once the water has pushed to the top of the radiator again.
Result!!! tighten the valve and switch on the heating. Check the radiator for any cold spots