Chop the chocolate finely, making sure there are no large lumps.
Heat the cream in a saucepan over medium heat until it is scalding. This means that there is steam rising from it and tiny bubbles appearing on the surface. Do not let it boil.
Pour the hot cream over the chopped chocolate and let it sit for 1 minute.
Use a metal spoon to stir in circles in the middle of the bowl until all the chocolate is melted and the ganache is glossy and smooth.
Let it cool to the desired consistency, depending on what you are using it for.
Notes
Make sure you use real chocolate, not chocolate chips or candy bars. The cream should be heavy cream, heavy whipping cream, or double cream - it needs to have at least 32% fat percentage.