>>85356836get familiar with the whole roster, mystery heroes is a good gamemode to practice this if its in rotation
map knowledge is also important, knowing where health packs are will help you if you find yourself in a bad position
find a couple heroes you're good at and stick with them. most players have a couple heroes they can play competently and can switch between as the game requires though some retards (e.g. me) just play one character and if you practice enough with that one character you can learn to play around your counters and still get value
keep sightlines in mind, for example if you're playing with an ana as support then you need to be aware of where she is or else you might push too far out of her line of sight and then she cant heal you and then you die
there's more but that should cover basics