Start with these fine tutorials by dartthfurby
Synfig Beginner Tutorial Series
[url]Synfig Beginner Tutorial Series: Updated 11/3/2013]
If your really ambitious and can afford it theres -
The first Synfig Training Package is available!
[url]http://www.synfig.org/cms/en/news/the-first-synfig-training-package-is-available/[/url]
Coming from gimp you may find these Tutorials helpful
[url]http://youtu.be/Q-Mf9hbft8E[/url]
[url]http://youtu.be/BWNzY_HZhEc[/url]
Bones are still an experimental feature in development as of this post check this out to see current status : Development: Bones (part 2)
[url]http://youtu.be/qbWT9sjh36U[/url]
Hope this was helpful.