Karen is of course absolutely correct in her listing of helpful books.
But you might also try ORB at:
Dave Postles has an online course in paleography that should help. For
me, the real key to mastering the art of reading manuscripts is
practice, and he provides the perfect vehicle -- interactive photos of
real manuscripts, over which you can move your cursor and see at the
bottom of the screen the transcription of each word. Deciphering the
abbreviations is just a skill, one you master by looking at lots and
lots of them. Enjoy!