The ideal way we are taught in school is to melt the chocolate over a double boiler. Sure you can microwave chocolate, but the problem is that if it goes for too long, the top overcooks and might turn lumpy and bitter, while the center is still solid. And worst of all, you HAVE to keep mixing the chocolate in the microwave in order to evenly distribute the heat.
Poohbearpenguin makes a decent explanation how to set up a double boiled, but some things worth mentioning. You should bring the water to an immediate boil first, then reduce heat to Low. This will reduce time you're waiting for hot water.
Dry the bowl so there is no water or moisture in it at all. Water will cause the crystaline structure of melted chocolate to seize and reform in lumpy masses. Wipe the bowl dry with a paper towel, then place chocolate in.
When water is at a simmer, place the bowl over the pot, and gently fold the chocolate on top of itself using a rubber/silicone scraper. It's important to evenly distribute unmelted pieces with hot pieces, and to scrape the sides of the bowl so the edges with smears of chocolate don't burn.
If water gets into the bowl, or if steam somehow condenses into the chocolate and it seizes, add 1 oz of cold unsalted butter into the chocolate, melt the butter in gently. After folding butter in if chocolate is still lumpy, keep adding butter in 1 oz intervals until it comes back