Since you're new and your mechanics/timing/priorities aren't really there yet I would generally recommend popping uber sooner rather than later. Better to use a bit early than drop someone/yourself and get flamed.

if you are low hp when you pop, you will still be pretty low when the uber fades and will be very easy to kill
ideally you want to pop BEFORE you are hit in the face with a pipe or a close range rocket not after, dont stare at your health waiting to be under 80 to pop or anything like that
better have a crappy uber than drop it :o besides in many maps you want to pop early in case of sticky traps or if a sniper is watching

only use kritz when someone screams at you to use it

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@ Finnegan This mentality can be helpful, but it can also be dangerous. Dropping an Uber and popping uselessly are essentially the same thing. You'll be flamed much harder for dropping, but you need to try and use your Ubers well.

This is why, when pushing, you almost always want your heal target to tell you to pop. Since they can see everything, and you can't, they're a much better judge of if it's a good time to use Uber. If your players don't remember to do this, remind them. You can use Uber if you see something that your heal target doesn't see, but otherwise, trust your teammates. If you drop there, it's totally not your fault, it's your pocket's fault. If you pop through every scary-looking choke you'll waste a ton of Ubers.

When you're defending and people are trying to force you, try not to pop against a single suiciding player if you have your combo (Soldier, Scout, +1) around you, unless you know 100% that you will die to the next shot. Your team should be able to kill the guy before you die, and if they can't, it's their fault. If they sac multiple people, it's fine to use Uber since you can get two kills off of that Uber, which is great.

What you're trying to do with an Uber depends on the situation. Generally, you want to Uber people so they don't take huge amounts of damage, because you generally want them to be good health in the post-fight.

If you have even Ubers and are exchanging with your pocket, it's okay to hold for a bit since you're tanking him for the entire Uber, so there's enough time for him to get fully-healed again. Your teammates shouldn't derp in and risk taking a bunch of damage and need flashes to not die, but if they do, you should still flash them, even though it'll make your Uber last shorter.

If you have advantage, then you don't need to worry about milking, since you don't have to worry about your Uber ending before theirs. Flash liberally to get people through the choke, catch out kills, and have good position and health for the post-Uber fight. Think about whether or not you can win the push without using Uber at all - if you can comfortably win a fight without popping, try and save it. If not, don't dilly-dally until their Medic manages to build his own Uber up.

If you're down a bunch of people then you either want to leave without popping and get a force off with your team, or trade Ubers and run away without anyone dying. If you're looking to trade, again, your pocket is largely in charge of deciding when to pop. Since you're numbers down, it's highly likely you're pushing into multiple people who can quickly do 200 damage to your Soldier, so be wary.

Kritz is good because it charges faster than Uber, but the actual charge is much worse than Uber. You want to use Kritz so you can Kritz on them before they get Uber, so the best time is when you'll be even Ubers with regular Medigun and have a small ad if you have Kritz. You need to be using pretty much as soon as you get it so you can take the fight before they get Uber.

It's also weak against Sentries and Pyros, which negate it, and Snipers, who just snipe you. It's also important that you don't telegraph by showing them your Medigun / heal beam, or have your Demo suspiciously peek chokes around the time you should have Kritz. If they know you're on Kritz, they can neuter it just by playing extremely aggressively while their Medic plays very safe. Since it's reliant on surprise, it's countered by a Spy disguise-checking your medigun, which makes it less likely to be surprising when pushing last, or if they have someone respawning who can quickly check that. You also don't want to do it too much or they'll suspect you're doing it.

Hello,

As ultimate said, wait for one of your players to tell you to use it. Make sure you tell them you will use it on their call.

Since you are beginning, use uber :
-when you are shot at and under 100 HP
-when your combo players ask you to use it. They should always tell you when to pop during pushes.

Ideally, no. If they are about to die force them to tell you to pop, or just run away saving the ubercharge.
However, since you're a beginner, apply the same rule as you do on yourself = pop when they are shot at and under 100HP, unless they explicitely tell you not to pop.

If bad ubers happen, this is not the end of the world. Keep the last 30% of the uber to put yourself in safe positions and let your teammates handle themselves.

GLHF

A huge thanks to everyone who commented on this thread. It really does help me a lot :)

Another few tips off the top of my head

Don't look away when you're being chased/bombed. Look at the player shooting at you so you know when and where to move, hit surfs etc

If someone is trying to force you, try to analyze the situation - how much ammo do they have left, are they putting stickies under you, are they in a position to do damage to you (a good thing to know is pipes do 100, rockets do 112 max, scatterguns do 105 max). The same thing goes for popping in general, if you see a bunch of your teammates that are in a position to take tons of damage, it's better to use to prevent that damage. If you see that they're not being shot at by a lot of different sources or if the enemy team is out of ammo, it's safe to milk for a bit.

Always keep a clear head and try to be a few steps ahead, try to imagine what ubering will accomplish and what holding it will accomplish

After ubering, keep your eyes open for teammates that are in risk of dying/taking fat damage so you can flash. Also, keep track of what your own team is doing and what the enemy team is doing so you can back off if the situation is bad and stay in and keep healing if you're winning the fight

Keep your ears open for footsteps/rocketjump sounds from behind you/other places that you can't see

Know what the sniper sightlines on each map are and learn which spots/routes are safe

Never lead pushes through chokes, make sure your teammates pass first and check for hiders/traps but don't stay so far behind that you're baiting your team or risk dying to flankers because you're too far back to get help

Heal people that are in a risk of dying, if you have a 90 hp teammate behind you not taking any fire and a 150 hp teammate in front of you that's actively fighting, make sure the dude in front isn't going to die before you turn back and arrow the low hp player that's in a safe position

If you're surfing, try to steer clear of walls so a soldier cant get an easy wallshot on you

Like everything else in tf2, medic is also a matter of having played through the same stereotypical situations a million times so you can read what's going to happen before it happens and know what's a good decision and what's a bad one

Good tips!

This is true, but remember that you run slower if you're only holding S. Alternate spamming a and d with a null movement script alongside holding s to keep your speed, or hold either a or d alongside s and walk diagonally.

Also Scattergun does 104 max.

Also it's a good idea to not go near walls in general when a Soldier is shooting you.