I'll start.
If you're tagged, what's the one piece of advice you'd give a hunter on his first year, with the intention of giving him the biggest heads up for the rest of his career?
I am not tagged but have killed around 120. Of those I can count on one hand my losses because I learned and changed after each. My main advise is learn deer anatomy, don't take shots you haven't practiced, and know the limitations of your projectile. Do this, and as far as the killing goes, you will be fine and rarely if ever lose a deer. Lastly, be very selective on who you listen too. If they don't have age and experience don't take their advise. This also means ignoring the guy behind the gun or bow rack unless you have thoroughly vetted them. I have been told and witnessed people get advise that will set them up for failure too many times at stores.
Anatomy:
"Behind the shoulder" is a dangerous term that gets deer lost. It is too ambiguous. Aim vital V always gun or bow. All the major killing stuff is upfront. Don't be afraid to go for it. Sadly, I see even national publications promoting shooting deer too far back. I had to endure a podcast today where four guys who are proclaiming themselves to be experts by hosting a such a program, couldn't understand why a certain shot placement yielded the results it did. I can't blame them too much when soo much bad info is out there.
Practice:
Shoot your gun at the ranges you plan to hunt so you know your zero. Check your zero a couple of times each season. Same with a bow. Practice from your stand or saddle at a 3d target as much as possible and learn your angles. After opening day I only shoot my 3d targets. I do take a small block on the road to check my sights, but my main every day practice is on a 3D. Pounding bag targets are for the preseason. Lastly, practice well further than you plan to shoot a deer, say 20 yards. You would be amazed how easy 40 becomes when you practice at 60.
Projectiles:
This is a biggie and the cause of a lot of lost game. Rifle hunters, USE HUNTING BULLETS, not rebranded target bullets such as Berger or Hornady ELDX, or Nosler Ballistic Tips. Bullets have recommended impact velocities. You should only take shots a distances that coincide with these velocities. If a bullet is too weak and deer pops up at 50 yards, your bullet can explode on impact wounding the deer. It doesn't matter if you have a 300 win mag or a 6.5 Creedmore. It happens often, and guys think they missed much of the time. Conversely, if you bullets is of too heavy construction and you shoot a deer 400 yards away it may not dump enough energy to quickly kill the animal if at all. Bullets kill differently than arrows. They require some shock to kill cleanly. Sticking with tried and true hunting bullets such as Nosler Partitions, Barnes, Federal Trophy Copper, Federal Terminal Assent, Remington Corelokt, or just about any bonded bullet will get your deer without fail. Lastly, shoot enough of a cartridge/bullet combo to get an exit hole on a broadside or quartering animal. It is a failure otherwise.
Arrows need to be well tuned. This is just as important with mechanicals if not more than fixed heads. An arrow flying crooked gets robbed of energy and doesn't penetrate. The res of this one is too complicated. I defer to Ashby's 12 rules
DR. ED ASHBY'S 12 ARROW PENETRATION FACTORS and also the Ranch fairy. Too many take these guys the wrong way. They don't say you have to shoot a 700 grain arrow with a 1" two blade. Study the rules and incrementally apply them to your setup. I did and couldn't be happier. I shoot a 455 grain arrow with 14.5% FOC, Sevr 1.5's or 1.75's for deer, Solid 3 blade for big pigs, @ 288 fps. I know the limitations of that setup and take my shots accordingly.