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 1 of Dubrovin, Fomenko, Bovikov, Modern Geometry.