Start with python, it is a much easier language to learn for a beginner and there are plenty of beginner tutorials on the net and almost all that is linuxrelated will work on the N900 too. (The N900 uses GTK for windowmanagement)

To learn C/C++ you should learn assemblyprogramming first. that will make it a lot easier to understand C since C only obfuscates the machinecode below..
( A simple tutorial on assembler, on a cpu wich relies heavily on pointers and similar C specific stuff, http://www.cosmacelf.com/shortcourse.htm )