How to choose a good grinder for a gaggia espresso machine? I am using a Gaggia classic espresso machine and am looking for a grinder that will work well in combination with it. 

What are the characteristics I need to keep in mind when selecting a grinder? What is better? A conical burr or a flat burr grinder? What gives the better grind? 

At the moment, I have a crupps, but it has very coarse grinding steps.
For espresso, you should use a stepless grinder (meaning, instead of a small number of grind sizes, you can fine tune the dial continuously) with conical burs (which are a certain shape of grinding wheels that provide consistency and precision). A standard recommendation is the Baratza Sette. The Reddit community /r/coffee provides more guidelines on shopping for coffee grinders.