I’m not an expert but this is where i’m at now and here’s some resources i’ve found on C (and C++) programming:

http://www.programmingtutorials.com/c.aspx

http://www.physics.drexel.edu/students/courses/Comp_Phys/General/C_basics/

CStandardLib Docs:
http://www.utas.edu.au/infosys/info/documentation/C/CStdLib.html
http://en.wikipedia.org/wiki/ANSI_C_standard_library

But I also bothered to get a copy of Sam’s Teach Yourself C in 21 days, a nice thick detailed book with exercises that get you to learn the concepts well(I recommend you don’t try to finish in only 21 days but take your time instead, seems more fun and beneficial to me, anyways), and it even includes intros to Java and C++…

It seems enough to have a basic understanding of C because quite a bit of writing externals is getting to know the specifics of the API described by the Max SDK. Eventually, the best thing to do is to take an example object’s XCode project and start examining and hacking away at it. One last thing, you will probably get more numerous and comprehensive answers if you ask this question on the "Dev" list(as opposed to the regular "Max/MSP" list) but here’s good, too. Best of luck!