Learning the geometry of the arenas is the first step in training your horse, for how can you teach your horse if you yourself do not understand?  Below are the two sizes of dressage arenas you may see at the schooling shows.  Learn the letters – one trick that Sue Hughes (long time Michigan dressage rider, clinician, English and Western judge) says all the time is this: 
A Foolish Boy May Catch H E K.  The beginning letters are for the small arena. 

The larger arena has more letters and is a bit tougher to navigate.  Movements that call out letters should be executed with your shoulder at a specific letter.  Practice makes perfect!  Read this ARENA ARTICLE for some good tips and tricks.