The easiest way to upload is to make a PDF out of it; most popular writing tools will have that as an output or export option. If not, you can easily make a google doc out of it and export to PDF from there! That'll make a file that can easily be uploaded to itch and then read by other folks. ^^
There are other compatible formats for different devices (like ePub for most tablet readers) but those can require specific tools to create. Still, it's worth it to make those extra versions for better accessibility if you can manage it. And itch will accept as many different formats as you want for any single thing you make, too! Best of luck! <3