Method #1: Bain-marie (my personal preference): Place a pot filled with about 1 inch of water over a stove burner set to high heat. Heat until it starts steaming, but not boiling, then reduce heat to low.
In a heat safe bowl, combine chocolate and heavy cream. Place the bowl over your pot of hot water, making sure the bottom of the bowl doesn't touch the surface of the water.
Heat the chocolate and cream, stirring constantly, for about 5 minutes, or until the chocolate is melted and the mixture is smooth. Remove from heat once done.
Method #2: Heated cream: Place your chopped chocolate in a heat safe bowl, and set aside.
Pour your cream into a medium sized saucepan. Heat over medium heat until it just starts to bubble along the sides. If it starts to boil, let it cool down slightly before adding it to the chocolate.
Pour your warm cream over your chocolate, and let sit for 5 minutes to melt. Whisk until your ganache is smooth and the chocolate is melted. If you have pieces of stubborn chocolate that aren't melting, place bowl over a bain-marie to melt the chocolate.