New player checklist before your first Mythic+ key

Bring consumables, know your interrupt, bind defensives, read the dungeon journal, and tell the group you are learning. Most early key disasters are avoidable with communication and a few keybinds.

Also learn which mob casts are stops versus interrupts. Stuns, knockbacks, blinds, and incapacitates can save a pull even when kick is down.

A defensive used at 70 percent health is usually better than a defensive remembered after the healer is already panicking.