You might have two tanks. One might be about 500mm x 300mm x 300mm and one might be about 1000mm x 700mm x 700mm
In each there is likely to be a ball cock like in an old WC cistern. It will be made of brass or plastic. If you are lucky it will be the rubber washer and plastic cone that are worn. In which case if you bale out the excess water into a bucket you might see it dripping. It is easier to fit a new valve than to tinker with the old one. You must turn the water off at the main first and verify that the dripping has stopped before you undo the valve. Often you can leave the existing stem attached to the side of the tank, undo the big nut inside the tank and swap the working parts. If you enjoy tinkering you can then re-washer the old one and leave it in the loft next to the tank ready to swap over in ten years time when it needs doing again.
If it is the small tank, measure the length of the old one before you buy a replacement, to make sure it is not too long to fit.
You need a "Part 2" valve which prevents pollution of the drinking water. They are very inexpensive. Brass is usually better. Pegler is a good brand. You can re-use the plastoc float, it unscrews.
If it is the small tank, then you have an additional problem since the drip will have washed out the anti-corrosion chemical that prevents your radiators from rusting through from the inside. Look for any labels or plastic bottles by it that give the name of the chemical last used. Newer boilers usually don't have a small tank.
If changing the valve doesn't fix it, the next fix will take a bit longer.
Some ladylike merchants use the term "float valve" to avod the embarrasment of saying "ballcocks"