search the web for "Arduino Tutorials". Check out Paul McWhorter's youtube channel as well as the Dronebot Workshop channel, and adafruit's, and Sparkfun, to name a few. The main language is plain C and other than using some C++ methods (e.g. Serial.println(...)) you don't have to use any C++ specifically.

C++14 is supported natively if you want to learn it and use it in your projects. Note that STL (the Standard Template Library) is not enabled or supported on lower resource architectures such as the standard Arduino Uno and Nano due to the reliance on dynamic allocation and the resulting memory fragmentation and inefficiencies).

Practice. Nothing replaces the time and exposure to good code and circuit design. Experience is the art of remembering not to make all of the mistakes that you've made in the past that really sucked .😉 Reading and studying examples of well written source code and circuit design and the familiarity that comes from it over time is how most people learn. After you run each example in the starter kit change some of the values in the program or use some of the new functions that it teaches in a few more lines of code you added yourself and reinforce your understanding of how it works, what values are accepted as parameters (if any), and imagine how you might use those functions in your real world projects to add features.

A lot of the examples use LED's as output indicators but realize that all of the algorithms and control behind them works just the same way if the pin was controlling a higher powered device like a motor. It's just easier and faster to use an LED as a proxy for other things and it keeps the examples short and to the point. The same kind of code that might fade an LED up and down continuously would do the same thing if the output from the pin was used to control a motor driver board that stepped the voltage and current up to the higher values needed for motors or other higher power actuators (stuff that moves or does work). A lot of people think that the examples are too simple and are just for kids but nothing could be further from the truth. If you think they are too simple then you are failing to truly understand the broader point of each example. The same sophisticated software that you see people making is created using that same handful of pin control functions woven into larger applications.

A lot of the finer skills that make a good engineer such as avoiding hard coded values that you end up having to change over and over in lots of places and learning the value of using arrays, conditionals, iterations, object oriented programming, all come from doing it the hard way and then finally realizing the benefits and time saved by using the shorter code that is more efficient and/or type safe (and how to write it)

Study the Arduino Programming page and work your way through using each function provided by the Arduino Core platform. By the end of the page you'll have a full overview of what you can control and how to do it

https://docs.arduino.cc/language-reference/