I'm on the Xbox Live version. Still haven't bothered to seriously get into the game and figure it all out.
Move the crosshair over people who aren't wearing the same color as you and hit the fire button.
Now the part people usually screw up is that they don't keep the crosshair on people! And thus they don't hit them! Much like Tetris, it is easy to learn but difficult to master!!
With my guide EVERYONE here will soon be a master!
The heavy wields the minigun, that sounds kind of small and useless! He is called the Heavy because he is a heavyweight champ! So just use your fists! Sometimes he will uppercut and send a foe flying! You are big and have lots of HP! This means you should have no trouble sneaking up on foes for that uppercut! Shout Rocky quotes every time you score a kill! YO ADRIAN! EYE OF THE TIGER! YER A BUM! JOIN THE NINTENDO FUN CLUB TODAY! MY STYLE IS IMPETUOUS! MY DEFENSE IS IMPREGNABLE!
Demoman is a difficult character. He is a depressed alcoholic who tries to kill himself at the start of a match while the medics try to stop him. Smash your bottle against a wall and it might shatter! Creating the omnigro knife. (His final weapon with a mysterious name) you can stab foes and deal massive damage with it now.
Scout is capable of double jumping! Using this he can easily reach locations others cannot and pick them off with his pistol. He has a baseball bat he can use to deflect grenades and rockets. He also counts as two people when capturing a control point due to his legally recognized split personality disorder. His super shotgun is like a shotgun except it came from a long destroyed planet. He isn't very good on offense though due to his low health. Scout is best played as the anti-soldier. BAT THOSE ROCKETS BACK AT HIM!
Soldier has a rocket launcher he can aim at his feet to jump higher! Once you are high up you can fire at foes below you with the shotgun! Use rockets from far away so you don't get caught in the explosion. Most maps have perches you can fire rockets from safely with a good view of the map. Sometimes the rockets glow blue, these rockets can propel you even higher!
Pyro has an axe useful for chopping down the engineers devices, and can use his flamethrower to warn allies when he is trouble. You can use the flamethrower to light up dark areas to help friends who are afraid of the dark but watch out not to ignite friends. His shotgun is his best weapon for people who are close since its hard for them to avoid it. Quarter Circle Forward and Fire also enables him to use a fireball.
Sniper excels at mowing down groups of foes with his uzi and slicing up close foes with his machete. He also has a rifle but a glitch causes it to not have a crosshair making it nearly impossible to hit anyone with. He is best played as a mid-range character swapping between his uzi and machete.
Engineer is a powerful character having not only a pistol but also a shotgun! His wrench can be used to repair things as well. He can build sentries that watch out for foes and fire at them but you need to watch them since they sometimes malfunction and explode for no reason. You should stand directly between the turret and any foes so that they don't shoot it and blow it up. you can also build teleporters to swiftly go between you sentries and the spawn rooms for more metal to upgrade your sentry. he can also build a dispenser that gives you more metal and ammunition while you are storming the enemy base. blow it up shortly after so that no enemies find it and know you are there.
Spy is the most advanced character of all. he can disguise himself as other people so that the enemy will think they are dealing with a heavy when they are infact dealing with a spy! He can also turn invisible for about 10 seconds at a time so that he can sneak up on foes and shoot them. he has a knife that sometimes can kill a foe instantly but there doesn't seem to be any way to rely on that. stick to the revolver and turn invisible when people see you. He also has a sapping device but it doesn't seem to do anything when used on people, likely another bug.
A Medic however breaks the rules of the entire game! He must shoot his own allies with his syringes! Use the medigun on allies you think are spies until you have the Ubercharge (Ubercharge is german for Spy Killer) KEEP IT ON THE SPY! He may go on a killing spree now but rest assured this psychotic glow will let all allies know where the spy is!!
Following this advice its pretty difficult to actually lose.
EDIT: Forgot to mention you guys shouldn't spread this advice around too much.