There are a lot of useful distraction-free writing tools out there. If you'd rather not download an app, ZenPen is a wonderful web-based alternative with a simple design and minimal clutter.
To use the editor, simply go directly to the ZenPen site. Erase the starter text and start writing your own. It supports basic formatting including bold, italics, quotes and links. You can set a word count target and save the whole thing in HTML or plain text when you're done. That's it. If you really want to minimise distractions while you're writing, head to ZenPen and press F11 to full screen your browser.