Some of the mechanisms are more important to know than others.  Although it is wonderful to know all of the mechanisms from your textbook, you should especially concentrate on:

- the addition of a molecule across the alkene double bond.  Know the explanation behind Markovnikov orientation.

- the substitution and elimination reactions that your book spent a whole chapter on were covered in depth for a reason.  They are a great introduction to the electron-pushing style of drawing a reaction mechanism. They are also a good introduction to some three-dimensional issues with reaction mechanisms.

 

You're going to need to memorize quite a few reactions.  Once a week from mid-October onwards in the first semester and a half-dozen a week in the second semester.  I recommend making flash cards and reviewing these flash cards daily.  I don't know any other way to keep all those reactions in your head.  You need to have a rapid, facile command of ALL the reactions simultaneously for doing multiple step syntheses.