Can I use the word processing program that came with my Mac?

Sure. It’s just sort of a Microsoft Word “Lite” product. In the end, assuming your book is straight text, you’ll want to save your manuscript as a Word .doc or .docx file or as a PDF file before submission. And remember, no extra lines, no extra spaces after periods and no tabs for indenting. If you can find a tool to help automate formatting, so much the better. You’ll save lots of time. Whatever you do, keep formatting as simple as you can. If you manage to format yourself into a corner, hire someone on Fiverr.com or Upwork.com, etc.  to help. Don’t let this process get in the way of writing and getting your book done. Done is better than perfect, although perfect is good, too.