First of all: You have done nothing wrong.
TYPO3 has it's own configuration language: TypoScript.
You have to tell the system in TypoScript what should be rendered in the frontend. The default is a simple TypoScriptSnippet that only prints out the text HELLO WORLD.
I looks something like this:
page = PAGE
page.10 = TEXT
page.10.value = WELLO WORLD
You can find the configuration of your page in the Template module in the backend in the field "Setup". You can either change the TypoScript directly in there or use external files and include them.
You have to understand at least the basics of TypoScript and Templating in TYPO3 to make the content you create in the backend appear in the frontend. If you learn from sources in the internet, try to avoid the old ones.
If you need more help, please feel free to join the TYPO3 community at slack (https://forger.typo3.org/slack), where you can ask any kind of TYPO3 related questions. We will happyly hep you to get started.