I'd recommend you go to cplusplus.com. Plenty of tutorial pages and topics to further your knowledge. However, be aware that Arduino uses a variant of C designed for microcontrollers (the exact flavour depends on the microcontroller type/family), rather than C++, so do be aware that some of the topics are not implementable on an Arduino. That said, I can't think why you'd be using some of the more elaborate C++ exclusive features in a fairly middle-of-road embedded system.
The only thing you need to ignore in C++ is memory allocation specifics - you're limited to the 'original'
malloc() based functions rather than the all-singing-all-dancing
operator new from C++. If you are in need of memory allocation functions, malloc is well-documented and a tutorial is only a quick Google away.
Of course I must also point you in the direction of the highly popular Stack Overflow site, part of this Stack Exchange network. While they don't have tutorials, there are plenty of great answers to common questions.