The most important tool is yourself. You are the writer, the one who creates. Prepare yourself.
In addition, anything that you can use to write is fine.
Pen and paper have been used for centuries and have been the tools of choice for many, even now.
The basic notepad has all you need to write a story and is even more convenient that the traditional pen and paper.
Word processors like Word and LibreOffice provides you with even more tools to make writing easier though they aren't really designed for creative writing but for designing the document and layout. You could write your story in them but they are actually for finalizing the document for (traditional) publishing.
There are dedicated writing software for the writing process like Scrivener, OStorybook, YWritter, and Manuskript.
On mobile, there are many writing apps.
But, in the end, just choose what works best for you.