There's no 'correct' answer to that, but I'd recommend reading each of the first three segments in chronological order before beginning the fourth segment in order to understand everything that goes on, similarly to the Avengers. Here are the three segments and their current chronological order of events and happenings (will be updated as changes are made):

Friendship is Magic:
-(none have been uploaded currently)

Equestria Girls:
-Stupid Girl
-Milkshake
-TFW You See Your Crush's Nudes

Dusk's Dawn:
-This Day Was Going to be Perfect