How can one play around with TypoScript code shown in documentations about TypoScript?
Tested with a local Typo3 test instance, Typo3 version 9.5.5, Official Introduction Package installed.
In the backend, create a new page in the tree. Rightclick -> enable it (the red overlay should disappear). Make sure it is selected in the tree.
Click on the blue Template button on the left. Usually it will show you the blue No Template message.
Make sure on the top Info/Modify is selected from the drop down.
Click the gray + (New record) button under the drop down.
On the bottom, click Edit the whole template record.
Optionally give it a speaking template and website title, like 'Playaround' and 'Play Around'.
Optionally, under tab Options -> Clear, select Constants and Setup - this will clear everything this Template has inherited from parent templates, but will also disable the debug infos at the bottom of the frontend for this page (if you have installed the Official Introduction Package).
Under the tab General -> text entry Setup, enter the following TypoScript:
page = PAGE
page.10 = TEXT
page.10.value = Hello World
press the Save and then the View button.
A new tab opens, showing you the frontend for your page, printing the text 'Hello World'.
Now you can play around with the code examples given in eg. the TypoScript Reference.