When you get in a pinch and need help with C4D, you can always check the documentation. If that doesn't answer your questions, the solution can often be found online.
Major video hosting sites such as YouTube and Vimeo have a vast array of tutorials that you can find with a simple search. If you think many people have had the same question as you do, odds are that using a search engine may yield a video or a site containing the answer.
That being said, here are some shortcuts, sites that I, along with other C4D artists, have found incredibly useful.
Homepage: www.maxon.net
Documentation: www.maxon.net/support/documentation.html
Python and SDK Docs: www.maxon.net/support/developer-support.html
C4D FAQ: www.maxon.net/support/faq.html
Maxon endorsed forums: www.maxon.net/support/maxon-related-links.html
One of the best third-party blogs about Cinema 4D and After Effects, full of great tutorials: www.greyscalegorilla.com
Cinema 4D Cafe has tutorials, forums, a gallery, and external links: www.c4dcafe.com
A site with great tutorials, tips, and a Q and A section: www.motioneers.net
A site that links to outside tutorials, but is also a fantastic reference for contemporary motion graphics design: www.motionographer.com
DeviantART has some great tutorials, and is also an excellent community site for artists who want to share their work or comment on work by others: http://www.deviantart.com/
There are some other commercial sites you can surf into:
A membership-based tutorial site, with some free tutorials as well: www.cineversity.com/
A pay-as-you-go tutorial site: www.cinema4dtutorial.net/
Official forums:
Unofficial forums:
For more open source information, follow Packt Publishing at http://twitter.com/#!/packtopensource