Function Writing Tools
You need only two tools to practice writing functions: a tool to write code, and a tool to run the code.
1. An editor or IDE to write the code
This could be something language agnostic like VS Code, Zed, or Sublime Text.
Or, it could be language specific, like Thonny (Python), PyCharm (Python), or WebStorm (JavaScript).
You could also choose an online option - either a full online IDE, or more of a sandbox or playground like CodePen or JSFiddle for JS, or Online Python or Pyodide for Python.
There are many options available. Don't limit yourself to the ones I've listed here. Look around for something interesting, then pick one and try it. Then try another one whenever you're ready.
You could also use a plain old text editor with no coding specific features, if you feel like doing things the hard way.
2. A compiler, interpreter, and/or runtime to run the code
Some of the above IDEs and playgrounds are 'batteries included'. You can type your code and then run your code in the same tool. In other cases, you'll write your code to a file with one tool, then run it with another tool.
For example, you might write your JS to a file named hello-world.js, and then run it with Node.js, like node hello-world.js. Or you might write a file named hello-world.py and run python hello-world.py. Or dotnet run hello-world.cs. Some languages will require a separate step to compile the program before you run it.
There are multiple options for some languages, and other languages have one single official tool. Find one that makes sense to you, but then experiment with others as well.
Where to go from here?
Pick an editor. Pick a runtime/interpeter/compiler or whatever. Then write a hello world.
When you're ready for something more advanced, check out advanced function writing tools.