On the aiming: You should first try out which heroes you find it easier to aim with, projectile or hitscan, then try using first the heroes of that kind for this matter. It’s also important to reduce your input lag (follow this thread ULTIMATE Reducing Input Lag Guide (SIMPLE) 🤓)
On hero balancing/HS: While the Hanzo problem happens a lot (getting him the nick of Spamzo), in fact good part of the random headshots will reduce as you get familiar with the maps, where each kind of player usually hits from, etc. Same happens with Moira. Key to deal with her is to not be alone if you can’t kill her fast. On Genji, unless you have a hero that can deal with him reliably, call for help.
Arcade: There are MANY types of arcade but I guess you are referring to Mystery Heroes, which is great to learn new heroes, but it WILL be a problem if you’re still starting and have no one to use VC to give you some tips on the heroes you don’t know. Ideally try to avoid Mystery Heroes until you at least have in your mind what good deal of the heroes work.