To get the most out of Python scripting in Cinema 4D, you'll want to have a solid grounding both in the Python language and in the Cinema 4D Python SDK. Fortunately, Python is a well established and widely used language, so there are plenty of excellent resources available. The Cinema SDK is still quite new, but also has some great resources. I've pulled together some of my favorites in this post.
In my very short career as a programmer, first for iOS and now Python, I've consistently found that starting with a good book is the best way to get up to speed with a new language. For Python, the book I recommend is Practical Programming: An Introduction to Computer Science Using Python. It's intended for students with no programming experience, so it starts with the very basics. The authors do an excellent job of explaining concepts and illustrating them with clear examples. It's also a very concise book weighing in at a little over 300 pages. The time you spend reading through it will pay you back many times over as you begin to write your own scripts. I highly recommend it to anyone interested in learning Python, whether it's for Cinema 4D or any other purpose.
Once you're comfortable with the basics of Python, you may want to dig deeper into the language. Despite its clear syntax and gentle learning curve, Python has some very sophisticated capabilities that will appeal to advanced programmers. The Python Essential Reference definitely lives up to its name. It's a comprehensive tour through the entire language, and covers topics including functional programming, object oriented programming, and many more. If you like the book and live in the Chicago area, you might be interested in taking a course from the author, David Beazley. He offers week long introductory classes in his Andersonville office.
If you prefer video tutorials to books, be sure to check out MIT's very popular Introduction to Computer Science and Programming. (You can also access this class through iTunes U.) This is a full semester course intended for students with little or no programming experience. The professors are engaging and clearly dedicated to teaching. You can watch the first few lectures which focus on the Python language itself, or keep going to cover more advanced computer science concepts like efficient algorithms for searching and sorting. (In the embedded video below, skip ahead to 16:12 for actual course introduction.)
The Python documentation is the official reference for the language. Cinema uses version 2.6.4. The documentation also includes The Python Tutorial. This is well written and covers a lot of ground, but it assumes you have some programming experience.
One of the first things you should do as an aspiring Python programmer for C4D is to create an account at Maxon's Plugin Café. This is where you can have your questions answered by the creators of the SDK and other Cinema 4D plugin developers. CGTalk also has a subforum for scripting and coding for Cinema. It has some very helpful tips (be sure to check out the sticky posts), but it gets less traffic than the Plugin Café.
CG Rebel has a couple of very good introductory tutorials. They describe the different ways you can use Python in Cinema 4D, and show you how to work with objects. They move at a good pace and are very thoughtfully put together.
The Third Party Wiki has several entries covering some of the most common tasks in Python for Cinema. The explanations are clear, and the code examples help pull the concepts together for new users.
Smart-Page.net has some excellent tutorials for advanced Python development in Cinema 4D. These tutorials definitely assume that you have substatial programming experience. This is a good place to look if you're thinking of developing your own Python plugins for Cinema.
908Lab offers some video tutorials covering Python and its interaction with Xpresso.
If you come across any other resources you like, feel free to share them in the comments. Happy coding!