I never had much success with the boolean modifier myself. What I did was Convert the font to a mesh. Alt+C, then select the sphere, then the font, and hit W and go to difference. If that doesn’t work completely select the font first then the sphere. Chances are it won’t work so you might be stuck doing it by hand… booleans are very difficult and never give me a good result. Good luck!
Retopo is a more advance type of 3D modeling where you add a plane and just extrude it around the original mesh. So basically you could make a “shell” of the font and sphere with another mesh. But that was it harder and probably won’t give you a great result at your stage. Here is a wiki article on it.
