What hero is easiest in overwatch starting out

Depends on what you want to do.

Mercy is like the go to beginner healer because she's very difficult to mess up at lower ranks and levels. I started on Mercy as support them moved to Moira. The concept of Moira is fairly easy but... I have an unpopular opinion when I say Moira is definitely not the easiest to play. You have to learn to balance both of your abilities because one is dependent on the other while your team depends on you.

If you want to play damage from the get-go. Bastion is a must pick on some maps. Stay off of the snipers unless you just want practice or have sniper practice from other FPS games. When I started DPS I played a lot of Junkrat and Pharah. Pharah is fairly easy once you learn how to time her rockets and Junkrat at lower ranks and levels is one of those, "throw your bombs and hope it hits someone" characters.

Tank... D.Va is fairly easy. Orisa is fairly easy. Reinhardt is rather easy but to efficiently use him to his max ability takes a little practice.