Is it difficult to start for baby?
you might want to elaborate your question - are you asking about soft way to mozart, teaching your baby to play the piano or just letting your baby appreciate and learn music?
HH posted this video in connection with this topic: http://forum.brillkids.com/teaching-your-child-music/soft-mozart-online-lessons-for-members-of-brillkids/msg34737/#msg34737 - i think that you might want to read this
you probably already realized that it would be difficult for the baby to play the piano unless there is one that is just right for his hands and your baby might not have the motor skills to be able to push the keys (HH says "23 months a little bit too young for such activity, but it is on the edge of being ‘just a time’… ")
i believe that just by playing good music (for me, it is mostly classical), babies learn and appreciate music - my son who is 1 year old loves music and “dances” to any music he hears (again, mostly classical) - he is even at the right beat.