I thought I'd write some simple tips for people that still have stuff to learn about TF2. My initial plan of attack is to write some thoughts about each class, in order.
The Scout is not actually a scout. If you use him to find out what's going on ahead of the team, you will probably die pretty fast (Unless you're using Bonk). With only 125 HP, the Scout is only two good shots of most weapons away from death.
Like most classes, the Scout's actual role is to do a lot of damage very rapidly. The Scout's Scattergun can do more than 100 damage in a single shot, which means he can kill almost every class in two shots. The important thing to remember is that this damage is only possible at extreme close-range.
Damage Falloff is a poorly understood concept in TF2. Almost every weapon in the game does less damage the farther away you are from the target. This is obvious with the Scattergun, since fewer of the pellets hit the target, but many people don't realize that the individual pellets do less damage at a distance too. For the Scout, the Scattergun is pretty useless once you get out of close range. If you know the target is near death, then it may be worth it to stick with the Scattergun, but it's generally a good idea to use your secondary gun if the target leaves close range. The pistol also has damage falloff, but it's going to do more damage than a Scattergun at a distance.
Every class's primary target is the Medic. Scouts can be excellent at targeting Medics since they can come from unusual directions thanks to double jumping, and they can move quickly in and out of combat. The perfect Scout suddenly arrives, kills the Medic with two close-range shots, then jumps out of sight. If there's no Medic to kill, then a Scout is best used as a cleanup force. While enemies at low health attempt to flee the scene of combat, the Scout pursues or blocks their exit.
Writing this makes me think there's a lot more basic things to talk about before getting to individual class tactics. I don't know what I'll write about next. Coordination with the rest of the team is the most subtle and important of skills in most FPS's, but it's even more important in TF2. That'll probably be my next subject.