How do I indent the first staff in a piece?

Load a custom style file for every new score that disables this, in Edit > Score > Style
noIndentOnFirstSystem.mss:

<?xml version="1.0" encoding="UTF-8"?>
<museScore version="3.02">
  <Style>
    <enableIndentationOnFirstSystem>0</enableIndentationOnFirstSystem>
    </Style>
  </museScore>

Same question had come up earlier today, see https://musescore.org/en/node/314934#comment-1067253

I would recommend not doing this via a style file. Most scores should have the first line indented; not doing so is more the exception for particular *types8 of scores. For example, lead sheets, or educational materials. So instead of forcing all scores to not indent, simply create a template - a normal score set up the way you want, then saved to your Templates folder - that has it turned off. For example, the Jazz Lead Sheet template included with MuseScore does this already, and I also have a "Handout" template I use for my own educational materials that also does this. That way I can easily create lead sheets and handouts that have it turned off, but normal scores will still have it turned on as they should.

You can do this by going to Format>Style>Score.
There is an option "Enable indentation on first system" Uncheck the box.