Let's get some basic stuff you should know out of the way first.

1) When you launch tf2 and click the "Play" button, you get a selection of game types.
Casual is unranked 12v12. Start with this. You can select gamemodes and maps when you select it.
Competitive is 6v6 and ranked, and isn't recommended until you feel comfortable with the game.
MvM is a coop mode, where you and 5 others fight waves of robots and use money you collect to get upgrades
The server browser shows all community servers. Many have custom gamemodes, so be careful when finding a game this way.
Making a server is only on LAN. That means people can only connect to it if they're connected to the same router as you.

2) So one you're in the game, some gamemodes have setup times. Use this time to place down buildings and engineer and possibly examine the map.

3) Weapon drops are random and there are VERY FEW that are direct upgrades. Just because you got a new weapon doesn't mean you should use it instead of the old one. Look at the stats and decide if you really want to use it.

4) You don't have to buy things. Every weapon in tf2 is avaliable through a random drop. Getting a Strange/Killstreak weapon doesn't mean it's better. It just counts kills.

5) Use the tf2 wiki: https://wiki.teamfortress.com/wiki/Main_Page

6) A warning about wearing the Ghostly Gibus: It is infamous for being a cosmetic that all noobs wear.

Don't underestimate the power of your scattergun. Two solid hits at close range can kill or severely damage any class.

For longer range or when you're scattergun is out of ammo, use the pistol. It's not quite as powerful but if you can land your shots, it can finish off a weakened enemy.

Just remember: you're advantage is you're speed and double jump. Use these to manuver around enemy fire and get close up to your target.

The rocket launcher on soldier is his primary weapon. Don't be shy to use it. In fact, use it all the time unless you run out of ammo and don't have time to reload.

Also, aim for the ground, not enemies. The splash damage will damage the enemy and it's easier to aim.

You can try to rocket jump too, but it's a hard skill that takes time to learn. I reccomend looking at another guide if you're interested in that.

You may have a flamethrower that lights people on fire, but remember to right click.
Rick click creates an air blast, which can deflect rockets (if you get the timing right), extinguish buring teammates, and knock people in the air, where they have no control over their direction.

Don't underestimate the power of the air-blast, and don't be afraid to pull out your shotgun ever once in a while.

Just don't W+M1. It's not fun for anyone.

Oh, and spycheck. You can see invisible spies if their on fire, and if you set a teammate on fire, it's definitely a spy. You're a spy's worst enemy.

Demoman is the only class with grenades. The arch is hard to get used to, but it's rewarding. One direct hit does 100 damage, and will also damage nearby players with the splash damage. Two hits can kill nearly every class.

You can also set traps with your sticky-bomb launcher. Put these near the objective (the cart in payload, control points, the breifcase in capture the flag, etc.) and detonate them when the enemy is near it.

Heavy is probably one of the more simpler classes. Hold Mouse1 to rev up and shoot, and hold Mouse2 to rev-up so you can shoot as soon as you press Mouse1. Just remember that you're slower when reved up.

Rev up while turning corners or capturing/protecting an objective. Remember to watch out for snipers and to spy check, since they're your worst enemies

Engineer isn't a very offensive class. Use him and his sentries to defend objectives, and use teleporters to get your teammates to the front line faster.

You build with your construction PDA, and you can demolish buildings with the destruction PDA.

All buildings can be upgrades to level 3. Sentries shoot enemies, despensers give you and your teammates metal/ammo/health, and teleporters, well, teleport people.

Medic may not be the most fun class, but it may be the strongest. You can heal teammates with your medigun, and if they're already fully healed, you can overheal them, giving them up to 50% more health than max (with the default medigun).

As you heal, you build ubercharge, which you can activate with Mouse2. It will make you and your heal target invincible for 8 seconds. However, you can't swich weapons or else the meter will continue to drain, but the effect will disappear.

Be careful of spies, as they often target medics. While ubered, you're still vulnerable to airblast from pyro and can die from falling into a pit or off the map. You can also heal and uber enemy spies, so be careful.

Sniper is the most basic class in tf2. He has a sniper rifle, an smg, and a knife. There's not that much special about him when compared to the other classes.

The few things you should know are that the sniper rifle does guaranteed criticals on a headshot, and that while you're scoped, you build charge, which increases the damage you do.

Just remember to check for spies and to not rely on your smg (it doesn't do much damage), and you should be good.

Spy is a very tricky class. You right click to become invisible and sneak behind the enemy. Use your disguise kit to look like the enemy, then stab them in the back. Literally. If you manage a backstab, you get an instakill.

Also remember that when disguising, a very noticable smoke effect surrounds you. This can be avoided by disguising while cloaked. If you need more cloak you can pick up ammo packs. This will become easier as you learn maps.

Make sure to avoid pyros and to act like the enemy. Don't run into enemies, since teammates don't collide with each-other, but enemies do. If you colide with an enemy while cloaked, you're cloak is revealed while touching them, and if you're not cloaked, they can notice it and will know you're a spy. If you don't do well, it's ok. It's a very hard class.

Oh, and you don't go faster when disquised as a scout. Sorry.