Be hyper aware of your surroundings, always keep track of your team's ults, and try to learn how to gauge enemy ults. You'll start to pick up which enemy has ult after awhile: watching who is doing the most damage, keeping an eye on the Sombra, etc.
Always watch your numbers when it comes to rezzing. If you can put the numbers in your favor, ie turn a 3v3 into a 4v3, it's a good call. Rez people with ults to clear or cap objectives. If you're in a team fight, use guardian angel every moment you can, don't stop moving. Only hide when there's an ult, otherwise stay with your team. Heal tanks to half and then heal DPS or others. Learn which tanks drop faster: keep an eye on your D.Va and Winston, Orisa if her shield is down. If you have a choice between healing someone with self-heal abilities and someone without, heal the person without.
Damage boost is your friend but watch it if the person is or could start taking heavy fire: people can drop quick. Damage boost, switch to heal, then back. If your sniper is good, help them get picks.
You'll get the hang of it after awhile. Her mechanics are super simple so you need to focus on game sense.
