I mained medic for a while, and he is 100% the most rewarding class in my opinion.
He is the difference between winning and losing in most Casual and Competitive situations.
What Medi-Gun should you run?
What you need to be able to do is know when you need to switch medi-gun, don't stick on a Kritz if you can't get out of spawn, switch to Uber.
If you're running Uber but you need to prioritize getting all your team to full HP, switch to quick fix as it heals faster.
Don't touch the Vaccinator because vaccinations cause autism it's not really that good of a Medi-gun to be completely honest.
What Primary should you run?
Crossbow. As of right now there is not really a reason to run anything other than crossbow, it has good damage, heals your teammates and charges uber by a few percent.
What melee should you run?
Ubersaw. There's not much else to run, it's by far the best, 25% uber on hit is huge and won't be beaten by any other saws for a while.
What should you expect?
To die. A lot.
If you main medic you are a high priority target for pick classes like scouts, snipers and spies, but most players will prioritize you first.
How to actually push sentry nests?
Do not listen to idiots that tell you to pocket Heavies, heavy can be a good pick, but if you're pushing a sentry nest your team can't get past, it's always better to pocket a Soldier or Demo.
How to be effective?
Do not glue your medi-gun to someone's anus, make sure your entire team is at full health, if not buffed.
Medic is a rewarding class, but it's very annoying when you're healing a scout and he still takes a med-pack that you could have used.
You will be the guy players love having on your team, but players hate when you don't forcibly glue your medi-gun to their anus.