Melting chocolate can be a difficult process as if it overheats (over 65⁰C) or comes into contact with moisture, the chocolate can 'seize', forming an unusable, grainy-solid substance.
Be patient and take it slowly. This is even more important with milk and white chocolates.
The best way is to use a bain-marie, a bowl placed on a saucepan over heat, with a centimetre or so of simmering water in it. Don’t let the bowl touch the water.
If you are not already using drops, break the chocolate into pieces, the smaller you break it the faster it will melt! Put this into your bain marie bowl and stir until melted and shiny. This can take up to 10 minutes, and you must keep stirring.