This site will NOT teach you basic Python programming for there are hundreds of excellent sites, videos and books out there that can help you with that. This site is mainly designed to help you learn about the extra commands that are available in EV3 Python that are not available in standard Python, such as commands to interact with the robot's motors, sensors, LCD screen, buttons and loudspeaker. However, I will give you a few words of advice on this page only about learning the basics of standard Python.
Before you study the robot-related commands that are unique to EV3 Python you should get a good feel for standard Python. To get familiar with standard Python you will want run Python on your PC - it is more difficult to work with Python on the EV3 because you have to communicate with the EV3 via a clumsy console interface. You will of course need Python version 3 on your PC (version 2 is now obsolescent) - I'm guessing it's already there but you should make sure and install it if necessary
To edit Python programs you can use any text editor but that would be a mistake - it's much better to use an 'Integrated Development Environment' (IDE) since an IDE will give you extra features like help with debugging and 'autocomplete' where you only have to complete the first few characters of a word and the IDE will guess the word that you are wanting to type, saving you a lot of time. The Python IDE that I recommend you use is PyCharm Edu, which is a free version of the very well-known and popular PyCharm. Some reasons why PyCharm Edu is perfect for beginners:
Only when you have become used to working with standard Python on your PC should you consider learning about the extra functions that EV3 Python offers - when are ready for that you should read this page.
Online Python Courses
I haven't checked which versions are used by the sites listed below. I am not aware of many Python courses specifically aimed at teens but there must be some out there - please let me know about any good ones you come across! Here is a selection of some of the best known Python sites: