Comprehensive Introduction to Competitive Overwatch

This twenty-page guide is an excellent introductory resource for players who are just starting with the Competitive mode. Although it is an older guide and some mechanics have changed -- the core of the guide is still extremely useful, providing a thorough walkthrough of basic concepts necessary to play the competitive mode. It's definitely a worthwhile read for newer players.

Overwatch: A Beginner’s Guide

A 71-page Google Docs document of all sorts of tips, covering a number of things beginners might like to know about Overwatch.