If you are going to do a lot of baking, a double boiler is a great piece of equipment. If you do not intend to bake a lot, it's probably not worth the expense and space. I do love mine and have had it for years.or a heatproof bowl (preferably a metal one) that fits on top of a pot of water.

Here's how a bowl and pot can work to do just as well as a double boiler (except you don't get a handle to remove the bowl.) Make sure the bottom of the bowl sits a few inches above the bottom of the pot, so that water can heat below and not touch the bottom of the bowl.In either case, the water does not touch the chocolate; the water is in a separate compartment below the pot or bowl holding the chocolate and the chocolate melts from the indirect heat created when the water below is heated to just below boiling and creates steam that is trapped in the space between the pot of water and the pot or bowl of chocolate on top. The amount of water should be enough to boil and create steam, but not so much that it touches the bottom of the top pot or bowl.

Although you may find directions for melting chocolate in a microwave, my advice is to avoid that method, as it is difficult to control the microwave temperature and timing.  That means it is too easy to burn the chocolate, and once you burn it, I don't think there is anything you can do to "bring it back."  Better to melt it slowly over a low heat on cooktop or stovetop, where you can watch it.

If you need to add a liquid such as whipping cream, it is important to heat the liquid first and add it to the chocolate.  In other words, you chop the chocolate into small bits or use chips in a heatproof bowl, heat the cream in a small pan until it is almost boiling (small bubbles around the sides, but no major bubbles), then pour the heated cream over the chocolate and stir them together slowly.  Do not add cold liquid to melted chocolate or it will "seize" and become a gloppy mess, rather than a smooth icing.