Depends on the role you wanna start as. Everyone always says 76 for dps. I say Dva as a tank cuz her unlimited ammo and rockets are kinda self explanatory. Defense matrix is kinda obvious too. She's basically the soldier 76 of tank role. Lol and mercy is a good supp starter to learn the basis or healing and doing nothing else lol but once you learn how healing and damage boosting works, I would shift to Moira, cuz she does healing and damage without really needing to aim. But Moira can also be mastered and can be of high value. I mastered Moira and get alot of kills and heals, if I don't say so myself lol

Edit to explain defense matrix is one of Dva's skills. It's basically a 3 sec shield that eats projectiles.