A couple of throughts from someone who recently moved from C (plain vanilla) to Ruby:
1] O'Reilly has a sale on C e-books on that expires today - have a look at them (http://shop.oreilly.com/category/deals/c-owo.do?code=WKCPRG)
2] Your biggest hurdles will be going from object coding to procedural coding, and going from dynamically typed to statically typed variables. Focus on these topics.
3] Once you have the basics of C, start focusing on dynamic memory usage (pointers, etc) as soon as possible. It's one of the most difficult topics (especially for someone that's only used managed languages), but also one of the most useful and powerful features in the language (and also the source of much pain if not done properly).
4] A possible way to start coding immediately, is programming C plugins for Ruby. MRI is very well suited to this, and will cover quite a bit of ground from points 2 and 3 above. There are heaps of tutorials available too.
5] As far as online tutorials go, there are many, and most tend to cover the same ground. Find one that you're comfortable with, and just do it. For the moment, focus on syntax and language elements, but DO NOT neglect to look into the fundamentals of how C actually works at compiler level at some point. It's an amazingly internally consistent design, and will make understanding and using some features a lot easier.
Good luck - hope this helps :)