the truth is, is that there are many different EDM's it seems like people create new styles of music everyday, also depends on what you want to listen to, also whether old school or new. Groups and such I recommend are
Trance/Eurodance/Techno-whatever you wanna call it
Paul Van Dyke
Ian Van Dal
Future Sound of London
Basically just start listening, soon you will find what you like best.
Also if you want to try a variety of electronica bands, try the Wipeout XL soundtrack, it has nothing but Euro-Electronica.
Oh yeah and just a personal opinion but American EDM sucks, its all about the Eurod EDM.