Scout:
- Get in close. Your scattergun does way more damage at shorter ranges.
- When retreating, take out your pistol. It can do some nice damage if you aim. That and your quick speed should get you out of scrapes.
- Use your double jump to reach good ambush spots.
- Stay away from sentries. Let the heavier classes deal with them. You're a lightweight harassment class.
Soldier:
- Aim at people's feet. Splash damage is better than no damage.
- Have a plan. When you fire all your rockets, either retreat to reload, or press on with your shotgun.
- Get good at rocket jumping. Crouch in the air and shoot a rocket at your feet, then rain death on your enemies from above. Nobody expects it.
- Use cover. Pop out to fire a rocket, then go back. It's the best way to take out Heavies and sentries.
Pyro:
- When in an open area, use your shotgun. It has a better range and deals good damage.
- You work best as either a hit and run or ambush class.
- Airblast helps out a lot. Use it when travelling with groups to keep your team safe from rockets, and knock people off control points with it. It can also put out teammates on fire.
- You are a Spy's worst enemy. Spray flames everywhere to root out the buggers. They can't cloak to escape you.
Demoman:
- Hold down the mouse button to charge up stickybombs and let them fly farther.
- Create traps by putting bombs on the inside of doorways where enemies can't see them.
- Bouncing grenades off walls is a good way to take out sentries without them shooting you.
- Aim well and be careful. You don't have a shotgun or anything to protect yourself, if you don't destroy your opponent with grenades, you're dead.
Heavy:
- Use your shotgun when traveling so you can shoot immediately if ambushed.
- Jump when going around a corner and spin up in the air to attack, you have good momentum that way.
- You have high health, but can't dodge damage very well. When your health starts getting low, get out quickly. Your health is your most powerful attribute.
- You can be a devastating ambush class with a bit of luck. Try to arrive unexpectedly and catch your enemies in the open.
Engineer:
- Stick near your stuff. There are a lot of opportunistic Spies wanting to sap it.
- Help out fellow Engineers, you can repair their sentries and build dispensers for them.
- Teleporters are important, set them in a safe location and upgrade them.
- Find a large ammo pack, and set up base near it.
Medic:
- Needles are hard to aim, but with your speed you can use them to take down a lot of opponents when there's nobody to heal.
- Go into advanced settings and play with the auto-call health percentages and the auto-heal functions.
- Uber the right person. Pyros are good for enclosed spaces. Demos are good for taking out sentries. Soldiers are good when making a push. Heavies are good for defending areas or making slow advances.
- Watch your back and your partner's back for Spies. Don't be afraid to use your uber early to save yourself.
Sniper:
- Be paranoid. You are a very easy target for Spies. Use your SMG to check for them.
- When traveling, take out your SMG. It's best for close quarters.
- A fully charged headshot can take out any class at any time. Scouts, Engies, Snipers, Medics, and Spies die from a single uncharged headshot or charged bodyshot.
- Change positions. After the deathcam, people will learn where you're shooting from and kill you. Use your team as a distraction, if they're not around, beat it.
Spy:
- Don't depend on disguises for too long. They won't fool people. Get in and get out fast.
- Don't wait for the perfect target. Kill someone, then cloak and do it again later.
- Stay away from Pyros.
- Your revolver is good if you get caught, but stick with your knife and sapper. Switch between them quickly to kill Engies and Sentries.
Other tips:
- Enable fast weapon switch. This is pretty much mandatory.
- Enable auto reload. Very useful.
- Enable the function that plays a sound when you hit an enemy, and shows how much you hit them for.
- Make a server and run around the maps to get a feel for them.
- Join a game and spectate to gain insights into how others play.