You can use any text editor that you prefer, however it is generally advised to not use Windows Notepad.

Here are some great ones that should be perfect for general scripting

• Visual Studio Code
• Sublime Text
• Notepad++ (Windows only)

Plugins aren't required to write scripts, but they do help by adding small niceties such as Syntax Highlighting.

• MrThomas' zenscript - Straight to the point Syntax Highlighting.
• Yesterday17's ZenScript - More advanced plugin that adds Syntax Highlighting, Code Snippets, BracketHandler Completion and more. (This may not be updated for new Minecraft Versions)
• Indexyz's ZenScript Syntax Highlighting - Straight to the point Syntax Highlighting.

• way2muchnoise's ZenScript - Straight to the point Syntax Highlighting.

• Eutropius225's Zenscript-Mode - Straight to the point Syntax Highlighting.

• Official Nanorc file - Straight to the point Syntax Highlighting.