Scout -> not enough experience
Soldier -> reload often, lead your opponent depending on the distance and their manuevering style so that the rocket hits their feet directly, do not try to grab ubers from classes that can use them more effectively, rocket jump into snipers' nests often
Pyro -> do NOT use backburner, always go for the puff+chop combo shown
here whenever possible, spy check ALL THE TIME especially around medics and engineers, do not throw away your life (it is more important than you might think) especially when an enemy uber comes charging through right over your dead body...
When it comes to stopping enemy ubers (by using the compression blast), depending on the situation, you may want to change the method. Against an ubered pyro/heavy/soldier, you will usually want to blast the uber target or both of them back (MAKE SURE YOU SPAM THE MEDIC CALL ASAP SO THAT YOU ARE HEALED AND DON'T DIE WHILE PUSHING THEM AWAY). Against an ubered demo, you should go for the medic only because the demo will rarely look backward and will find it especially difficult to kill you (given that you're careful) before heading on; the other classes will either see you blast the medic or can kill you more easily. Be warned that a good demo can and will destroy any guns he can possibly launch projectiles at; he has to be blasted extremely far away by the guns or he has to be zigzagged by your puffs.
Demoman -> 3 well-positioned stickies will destroy a maxed out sentry. use 4 if your aim is not so good; you can still get 2 guns per uber, 3 if you're good or lucky with a crit pipe/sapped guns. 4 if the engies are stupid and place them too close to each other. In a one-on-one battle against a _good_ engie with max sentry and you have cover and/or an uber, detonate your stickies at 1 or 2 down, or spam a lot around the gun. An engineer can destroy the stickies you lay down with one shotgun blast each or better, depending on the placement, so you want to either kill him first or make sure he can't destroy them all. That, or use your pipes first to encourage him to keep his wrench out (not ideal, though)
Demo can also be extremely effective proactive OR reactive uber defense depending on local layout. Lay down 4+ stickies directly in the path of the ubered pair and detonate when they walk across them. Ideally, you want to separate the pair so that the target loses his invuln, and the medic is too far away from anybody to re-target his uber, but just putting either/both into the air is effective as well if there is a maxed sentry hitting them (keeps 'em in the sky). If there is no gun, you REALLY want to blast ONLY the medic. You will get a feel for how the stickies cause enemy bodies to behave, so don't worry too much about perfect play the first time around. Oh, if a pyro sees your stickies and blows them away, DO NOT explode them unless there is no chance that either of the pair will walk over them. The medic sometimes does not see what the target sees (though the medic should always be the spotter).
On defense, you must keep snipers's nests and the like clear with your projectiles. Sticky jumping up and stickying doorways (anywhere with traffic, actually, not just doorways and not just after jumping into enemy territory) is also ideal. If you happen to get a crit sticky behind a doorway, you must blow it immediately because the sparkle can be seen through the wall.
in one-on-ones against other demos, don't hesitate to pull out your bottle at close range. you can try your luck against scouts, soldiers and pyros, but never against heavies. btw it is important that you practice your melee; comes in handy especially as a medic. in medium range, use your pipes. you have to lead your opponent so that they detonate on direct contact. once you run out, switch to stickies and "box" your opponent in. lead the opponent with the first shot (it's ok to overlead). you will notice that they run away from the sticky. your second sticky will again lead them. your aim is to create a deathtrap for the enemy whose stickies must explode as soon as you are reasonably sure the baddie will die. do NOT explode your stickies one at a time UNLESS they are low-HP classes or have low HP. do not use this tactic against pyros because they will just make a beeline for your ass. if they do not have backburner, run facing AWAY from them and aim your stickies straight down, then detonate ASAP or when you think they will have walked over them (do not wait to lay down more than one at a time; remember, fire is on your ass). jump before detonating. if they have backburner or are medium range, do the same but run away while facing them; easier to adjust optimal dispensing/timing.
as for fighting heavies with stickies, you should either be really far away or have cover (lay one every time you pop out). you should never be in a situation where you're close to a heavy, you'll die for sure unless you're ubered, but a heavy is generally not your best target when you are.
Heavy -> Not much experience, but you want to stay in areas where the only type of encounter is extremely close-range, unless you are backing up friendlies with your Natasha. Make friends with medics and make sure NOTHING passes you - not spies, not pyros, nothing; the medic is 4 times more valuable than you are.
Engi -> ugh, way too much that you need to pick up, and a lot of it is situational
Medic -> stay BEHIND and AWAY from your target. overcharge people whenever possible but do not let low-healthers stay too long like that; they tend to die and you don't build uber as fast. guys on the front line move up in terms of priority even if e.g. they have more health than a sniper behind you. it is IMPERATIVE that you do not risk yourself to protect a suicider. constantly scan 360 degrees to find people with low/non-full health, keeping in mind priorities, and also so that you don't get backstabbed. if a friendly seems to be homing in on YOU, especially when you're constantly moving, he probably is - to put a knife in you. suspect EVERYBODY. also, if you turn around and notice somebody behind you that you haven't made a note of, you have DONE SOMETHING WRONG. you NEVER let somebody near you without your knowledge.
equip the blutsauger and the uber saw ASAP. the needles are used for escaping and spy-checking; when escaping, run backwards toward friendlies while facing the enemy and shooting; the needles tend to "scan in" on the target and you prolong your lifespan due to the health imparted by the needles. spy-checking works either by showing the spy when he gets hit or when you notice you get a green +3 health from thin air. the former is more effective because the latter generally does not tell you where they are. as for the uber saw, only in extreme circumstances should you try to melee an offensive class (demos excepted). it is effective against spies and for spy-checking; you get +25% ubercharge from a hit on an enemy even if he is disguised as a teammate. also can work on other medics, stupid snipers, sometimes engies
about ubering: you must bail just before an uber runs out if you feel that your target cannot keep you alive until you get back behind your lines or unless you must keep him alive for as long as possible. avoid pyros and avoid walking over stickies. if your target is a slow runner or if there are many guns, you must run in front of your target in order to increase his running speed (guns push you back and if there are more than one hitting your target heavy, he ain't goin' nowhere). DON'T JUMP IF YOU ARE UBERED OR ARE UBERING.
Sniper -> Priorities are medics, snipers, heavies in that order (depends on situation and where teammates are). Fully charged body shot will kill a non-overhealed medic/sniper/spy/engie. If there are two medics close together, hit the one that's not "sticking" to a buddy; he's more likely to not be overhealed.
Spy -> Priorities are teleporters, engineers, sentries, medics, heavies, generally in that order, but it's kind of hard to take out a teleporter without killing the engi first unless you spam all 3 buildings and he has no backup. best way to take out a setup is to backstab the engi and sap the gun right away, then everything else. if you have the time, wait for the gun to spin maximally away from you before stabbing and then run OPPOSITE the direction that it spins to home in on you so as to prolong the time that you have before it starts firing. if the engi has backed himself into a corner, you can jump on top of his dispenser and then on him. note: you can jump on the gun, but you will die if the gun fires on a teammate. be VERY careful not to touch the engineer before you land on his head, because he will see your cloak partially fail (happens when you bump into enemies). try your best to "backstab" him while above him (face the same direction he's facing and point 45 degrees down, i guess) success rate is 50-75%. if conditions are ideal, meaning that you can figure out what timing gives you the most leeway for sapping the gun after attacking (which causes undisguising), you can use the ambassador to hit the engi in the head then in the body and THEN sap the gun (only do this is you cannot backstab and you don't want to topstab). i must note that i have never tried this myself because things never work out to be best case.
equip the ambassador asap unless your current mission is to destroy blue teleporters. will explain later