The book of  Gelfand and  Fomin  is a good place to start.  It worked for me. I would  like to include another  nice and short source  namely Chapter 19,  vol. II of *Feynman's Lectures on Physics*.

If you know a little about smooth manifolds, then Arnolds's *Mathematical Methods of Classical Mechanics* is another excellent source.       Also, check volume in of Dubrovin, Fomenko, Bovikov, *Modern Geometry*.