Well heres a guide to hopefully lessen the suckage of scouts. Scouts on our server have the least number of kills next to medics. Many scouts I see play to the class' weaknesses rather than strengths, so hopefully I can shed some knowledge on how to play this very frustrating class effectively. Tomorrow or some time this week, hopefully I can post up a demo of a good round of me playing scout. Seeing it is a lot better.
When to play scout
Its important to play the kit on maps that are kind to scouts. In some maps, although its possible to play the class effectively, picking another class will always be more beneficial to the team. The following maps are not generous to scouts because they often develop into drawn out stalemates resulting in the entire enemy team being within close proximity of eachother, or are spam laden resulting in the weak scout's death.
Dustbowl, Aqua, Hydro (except for the more open areas), Goldrush (2nd and 3rd stages), and Labor.
Scout's purpose
Although the easy answer to what the purpose of the scout is is "to capture points", thats hardly it all. Anyone can capture a CP pretty easily, and often stronger classes are better at it since its so easy to kill the weak scout off a CP since hes mostly stationary while capping. In CTF, spys are often better at capturing intel in the middle of the playing field due to sentry guns and what not (spy creep). The real purpose of the scout, as I see it, is combat. To 1) Kill isolated enemys, 2) Kill unsuspecting enemys, and 3) Cause disruption. To fulfill these purposes, the scout has to be a master of picking his battles and knowing when to get out of them.
Fighting effectively
When you are about to fight, make sure you are above 100 health. If you are ever below that, run to get health because you can easily die from any number of things below 100.
Once you come out into the battle, avoid large groups of enemys, especially if they see you coming. Look for the easy kills, being engineers, obvious spys, and snipers. Use your speed to get to the side or behind them and only fire when they see you or you are at close, lethal range--whichever comes first. In any battle against any class, look for height advantage to get the jump on enemys, attack enemys who are reloading, retreating or otherwise incapable of defending themselves effectively. Make sure every shot counts as the scout probably has the lowest damage per second of all of the classes. Personally, to make sure I dont waste scattergun shots when shit hits the fan, I focus directly on the reticule and try to block out anything thats not pertinent to the person im trying to kill or to my life. Just making sure you are focused on the opponent entirely rather than absolutely everything on the screen will make you more accurate. Its almost as if everything slows down a bit and you can more easily be accurate. Finally, learn to gauge approximately how much damage you have dealt (this only comes with experience). If you learn this, you can more aptly determine if sticking around will get you the kill, or leaving to fight another day is best.
I mentioned to avoid groups, but its important to reinforce this. You are so weak that if you give 2 or 3 or more enemys an opportunity to hurt you at one time, you will die faster than you can probably even hurt one of them. Along the same lines, avoid areas where a heavy amount of rocket/demo/etc spam happens, because you only need one thing to hit you to either kill you or force you to retreat to find health.
Specific strategies against other classes
If you are attacking a medic, once his medi target turns to face you get out of his line of fire. When he returns to fighting your teammates, jump back in. If you maintain your health appropriately, you will either kill the medic or your teammates will take advantage of the distracted medic target. Sometimes the medi target wont even notice you are there before youve killed the medic
Against demomen, if you catch them by surprise then the battle is easy. There isnt much he can do but get a lucky pipe bomb on you. However, if he sees you coming he will likely plop down stickies to disuade you. If there is no way to get around the stickies without getting hurt, then just avoid him--it isnt worth it.
Although it may seem odd, one on one, scouts dominate pyros. If the pyro is aware of your presence, keep yourself just out of flame range. Although you wont do lethal damage, you still do enough damage that after a few rounds he'll go down and meanwhile you wont take any damage. Pyros, realizing how futile it is to keep attacking with the flamethrower, will often take out their shotgun. As soon as you see this, jump right into his face and get a lethal shot, and them immediately get out of flame range as he will probably switch back to flamethrower. As long as you keep a watchful eye on how far you are from him and what gun he is carrying, lone pyros are often an easy kill.
Scout vs Scout battles are just clusterfucks. Its 90% luck, 10% skill. There isnt much to say other than make sure that every one of your shots hits, and pray a little bit.
Heavys will kill you easily with the minigun. Avoid hitting them at all if they have a medic or have their minigun wound up. It simply takes way too many shots to down a heavy, and it takes very few for him to kill you. Only ever go after a heavy if he does not have a medic, is not wound up, or you know that he is very weak. If you have to take down a lone heavy who is wound up, wedge objects between yourself and him. Pop out from one side, take a pot shot, and repeat on the other side. Change it up sometimes by popping out the same side twice in a row to keep him guessing and you MIGHT get the kill. Generally, just avoid him.
Soldiers. Uhg. These guys will dominate you. I know personally as a soldier, scouts are usually trivial kills. There are a few things you can do as scout to swing things in your favor. First, as with any opponent, try to close to lethal range without his knowing it. If he does see you, its often best to avoid him. However, if you notice him reloading his rocket launcher or he has his shotgun out, strike him very fast and close. If he doesnt have 4 rockets in the chamber, he will have a difficult time killing you. Since soldiers need splash damage or height advantage to kill anyone, make sure they do not have that advantage by your choice of where to attack them and how. Look to fight them near water or planks (well, 2nd/4th CP has several flanks on the second and third floors that are great for this), because you can easily jump out into the middle of air, get a shot, and then land. You will not take any damage while you are dealing damage. Always look to hit soldiers (and really, ANY class) from above. Jump down on them for a few quick lethal shots, and they wont have enough time to rocket you to death.
Overall, the most important things to remember are to always enter battle at full or close to it health, initiate an attack without the opponent aware of your presence, attack lonely enemys, and capitalize on opponents reloading or otherwise incapable of attacking you effectively. Think like a guerrilla.