Priorities change depending on situations. Obviously you want to keep everyone topped off, unless you have your ult. Then let the other healer heal to get their ult. Damage boost people as filler. Damage boost also helps other people get their ults. So if you see a mini d.va out of mech, damage boost her to get her into mech. And revive when it is safe and possible for you to. If the genji or tracer dies in nam, they stay in nam. You'll get used to playing mercy the more you play her. Good luck and have fun! Don't let the haters get you down.