Step 1: Download and install the Battle.net desktop app, if you haven't already, and launch it.
Step 2: Create a FREE Battle.net account, or log in with an existing account.
Step 3: Click the "Overwatch" icon, located on the left-hand side of the Battle.net desktop app.
Step 4: Select your region from the drop-down menu, then click "Install" to begin the installation process.
Step 5: Once installed, click "Play" to begin!