The important thing here is to remember that a compass points not to true north (e.g. the north pole, the geographical northernmost point of the Earth) but to magnetic north (the magnetic north end of the Earth's 'bar magnet'). These two are not in the same position (indeed the position of magnetic north varies with time).

The apparent position of magnetic north will vary according to your location in the world (most importantly your latitude) and you will need to know the difference between these two positions (magnetic and true north) to take an accurate bearing. Information on the deviation between the two will be found on any (Ordnance Survey) map, and should not be assumed as it can vary by a relatively large amount according to your location.

If the deviation is not given you can find it from the Pole Star or by using the watch method to point to north. Then lining up the compass with the grid lines on the map you can discover the variation if there is one.