When it arrives ‘soon’
When Overwatch launched yesterday, it had all of its planned features except competitive (ranked) play. That was going to be in, but based on beta and high-level player feedback, it was removed in its current state.
Speaking to the changes on Facebook, director Jeff Kaplan stated that they want to change the season length (one month, Hearthstone style, was too short for Overwatch), and will probably settle on “about” three months. Ranked play also might cut down on the amount of sudden deaths, and bring new gametype presentations (possibly new routes or level support)
Kaplan and the team is hoping to get it out by late June. Until then you can play unranked to your heart’s content.