The first thing you need to know is that Arduino sketches are C++. You can find documentation of the standard Arduino library functions as well as some basics of general C++ here:
And tutorials for the example sketches found in the Arduino IDE under File > Examples here:
But because it's C++, you are not restricted solely to the information out there specifically about programming for Arduino. You can find a huge amount of information about general C++ programming. You do need to be aware that much of this information will assume you are writing the code to run on a computer but we are writing code for a microcontroller which has limited memory and outputs so you need to be able to filter the useful information from that which is not applicable.
I found this to be a good introduction to C++ programming:
It starts with the basics. I'm not sure I would recommend just reading straight through it since some of the information in later chapters is on more advanced topics that you don't necessarily need to know as a beginner. So feel free to say "good enough for now", then come back to the tutorial when you're ready to learn more. I'm sure there are plenty other good C++ resources. That's just the one I happened to find when I needed it.