Scout:
Use the double jump. Noobs dont. Dont be a noob.
The Force-A-Nature's third jump is extemely usefull. give it a shot.
The sandman's -15 health debuff is not important. 15 health isnt going to help you much.
USE THE PISTOL! Most people udnerestimate it, but its extemely usefull.
Pyro:
Use the shotgun, not the FlareGun. A pyro is designed for close combat. The flaregun isnt. A shotgun is.
Dont W+M1, it usialy results in death. Of you.
Think about ways to get Behind the enemy, especialy with the Backburner. You are much more effective.
Dont forget to Air-Blast, it is quite handy in a soldier spam environment.
Spy:
Never. EVER. Discuise as a scout or heavy. The heavy is too big slow, and that just means more time for a enemy to shoot through you. And the scout discuise will not increase your speed- making you stick out like a sore thumb.
Dont forget to use the revolver, it is actiualy really powerful. Of course, use it as a last resort.
Dont bother saping engeneer's buildings unless they are on the front line. Stab the heavy first, let your team move up, THEN sap the damned sentry.
Heavy:
Unless you are extemly good at heavy, or facing a bad team, expect to die. alot. The heavy is weak against almost every class- A Scout will sandman, a Sniper will (easily) Headshot, a spy will stab, soldiers can actiualy aim at something other than the floor... the heavys large size and slow movement are its biggest weakness.
Dont bother with Dhalocks bar. Get Sanvich or the shotgun.
GRU Gloves are your friend.
Engineer:
On every map, there is one or two places to place a sentry that will block a chokehold. On the table in 2fort's intel room or outside the spawn room, for example. One engie will easily defend such a place from a mass invasion of a enemy team- if the engy has help, that is.
Dont ever, EVER, bother making a sentry next to another engies sentry. In 2fort, dont have more than one Sentry outside the spawn. Dont have more than one outisde your spawn. One sentry is enough, and you could be doing something constructive.
Dont ever- EVER- Set up a sentry where it can be outranged. dont get up on the battlements in 2fort, or on the Cart tracks in a payload map. It will be shot down by a soldier when it cant shoot back.
*SHOOT MEANIE, REPAIR TOY* when a spy saps your building, KILL THE SPY! Dont stand there like a lemon and try to repair your stuff, it will die faster than you can repair. Kill the spy first, then repair.
Move that gear up! If your team has several engies, and you insist on not switching class, move up. Make suicide runs to the enemy base with your sentry. The worst that could happen is you die, and you respawn. The best that could happen is that you set up a level 3 sentry outside the enemy base, and get easy kills. Of course, make sure your base is defended first.
Dont underestimate the Gunslinger. A mini-sentry is just as usefull as a normal one, especialy on CP maps and when on the attack in payload maps.
Use the normal shotgun. Its better than the Frontier Justice, simply due to its large clip size. Use it against spys, its more than enough.
And finaly,
HATS. DONT BOTHER WITH THEM. THEY ARE A WASTE OF TIME. ALLWAYS. DONT BOTHER WITH THEEEM!
Thats my general hints for my most played classes. Of course, these may not suit you well, but thats what i use, and (Not to brag or anything) its not uncommon to see me as MVP.
These wont apply to every situation, but these hints are a general, day to day basis of how i play. Good luck.
Youll learn TF2 quick enough- its not too hard.