If the head broke there shouldn't be much pressure on the bolt and it should come out pretty easily with a simple screwdriver (might have to notch the top). If it's not that simple, and there's enough bolt left, take a pair of vice-grips to it. If all the above isn't possible you'll have to just drill a pilot hole and use a bolt extractor.

Note: If the bolt is stuck chances are you're using the wrong bolt (ie: thread pitch is wrong, length is too long, or you've completely messed up and are using standard instead of metric).