The first step should be pretty obvious. Get Overwatch if you don't have it, then start up the game.
The second step is to start a game and then pick Mercy. If someone picked her already then either pick a different character and wait for them to change, or choose a different game and pick mercy immediately (she's in the support class, which is on the end of the character selection screen next to the tanks.)
The third step is to make sure that when you play you heal every teammate. When a teammate is at low health there will be a sign showing on your screen. The game makes sure you can heal them by pressing the shift button and flying over to them. Also if you press right mouse button or RMB with your main the healing staff, you can boost up their damage. If you want to heal them press LMB. Mercy alternative weapon does damage to your enemies. Also if you are high in the air press space to slow down (this is useful if you have to heal Pharrah, a character than can fly on her own.)
The fourth step is to not try to play offensively, a mistake made by many new overwatch players who try out support classes. Your secondary weapon is for defense, or if no one needs healing on your team, not to go ahead and act as if you're another character like hanzo(defense), reaper(offense), or reinhardt(tank). Also mercy naturally heals herself... do not abuse this! Don't think you can go rushing because of self heal. You only heal after a very short while of not taking damage. Be careful about every one of your actions as her. You are the backbone of your team as her. Try to not die at all. It is hard but the maximum deaths you should get as a pro level mercy is 1. Also your ultimate should only be used if a lot of people are dead, not because one soldier 76 or reinhardt died.
The last step is to simply follow the other steps and practice them every chance you can. I will now show a video of someone who is a pro mercy player. Make sure to watch how he plays mercy because this is an example of playing mercy well. Ignore the voice lines those aren't needed to play well
This is the end of my presentation. If you want to see more gameplay of overwatch go to gameplay only, or other overwatch channels. Also this goes the same for every console you play Overwatch on. But the RMB or LMB is only for pc. Google the controls for the game on console. Thank you for reading and have a good day.