The most basic bits about working in Jupyter are covered in the example below. For clarity, the example refers to the menu commands, but there are very useful keyboard commands to discover for all steps here.
Create a notebook
Click 'New' to create a new 'Python 3' notebook. Click 'Untitled' at the top of the notebook to change its name to Hello World.
Insert a markdown cell
Menu: Cell > Cell Type > Markdown, to make the first cell into a markdown cell for inserting text or other descriptive content. This is written in markdown code, which converts when running the cell, by pressing the 'Run' button or control+Enter.
Insert a code cell
Menu: Insert > Insert cell below, to insert the next cell. Don't change this one to markdown, but keep it as a 'code' cell, and write some Python in it:
Once again, press 'Run' or control+Enter to run the cell.
Click the Jupyter logo to go back to the root of the folder to see your created notebook saved there. Click 'Upload' to upload some other Jupyter notebook to run or edit (file extension *.ipynb).
This concludes my 5-step tutorial on getting started with Python and Jupyter. You are now all set to write Python code, and to run it, both in the Atom editor and in Jupyter notebooks. The stuff covered will have lifted you over the first threshold into Python programming. With these things set up, you can download, edit, and run code created by others. You also have the necessary infrastructure to easily deploy all the stuff you will learn from the vibrant Python/Jupyter community online.