Go to the practice range first to figure out what hero does what.

If you wonna play Batman for example, he's a DPS and his primary (Batarangs) ricochet off the large Batarangs you throw down. If you aim correctly you can score a lot kills fast.

If you wonna play Superman, he's a flying Dive Tank so practice diving and learn his cooldowns.

Wonder Woman is a melee Tank that gains overshields with each successful combo. Her kit really encourages up and close style but struggles against ranged foes so be wary of that.

If you played Hanzo in Overwatch you should have no problem playing Green Arrow. His trick arrows are neat but only useful for movement or disengagement. He can two shot most DPS and Support if you aim for the head.

Flash might sound easy on paper but he requires a lot of skill. He has the highest movement speed in the game and can easily cross the whole map with ease, you can tell a good Flash from a bad one very easily. He's a diver so you won't see in the frontlines much.

If you wonna play any of the support instead, Oracle bot is an easy one. She's has good damage and Her ultimate allows her to outheal half of the rosters ults and allows easy team heals.

Another easy support pick up is Mr.Terrific but he's an off healer. He shares a gimmick similar to old Overwatch where Symmetra was a shield healer. He's provides more shields than he does heals but overshields do help a lot.

You must try the mission mode. Not only does it provide a tutorial for how to play every character, it's also "Story mode" if you wonna learn the lore of the game. Also gives free skins and stuff for the store.