For a first look at the capabilities of TeX, click on the "TeX Showcase Leaflet" and "Welcome to TeX! Now What?" links. TeX is very versatile and can be used to typeset beautiful documents in many languages and in different fields (mathematics, astronomy, physics, chemistry, biology, linguistics, etc.). Samples of TeX documents in these fields can be found by visiting the TeX Showcase webpage.
If you have used TeX or LaTeX before, no doubt you have a favorite editor or front end. Configure it to search for binaries in /Library/TeX/texbin and start using the distribution. If TeX is brand new for you, go to /Applications/TeX and read the document READ ME FIRST, which explains how to find and use LaTeX. A copy of this document is available here.
See TeX Resources on the Web - TeX Users Group for a page concisely summarizing the vast web resource material about TeX. In particular, to learn to write in TeX you need to read an instruction book. TeX is the same on all platforms, so any book on TeX will contain useful information. Some of the books commonly recommended are listed on TUG's TeX resources page.
The LaTeX Project's own step-by-step tutorial is available at
Happy TeXing on macOS !