As a soldier you should stay on high ground and aim at enemies, aiming at their feet is easy. Shoot their Torso's if they arn't moving.
As a demo on Defense you should use the sticky bomb launcher to place stickies(bombs that stick to stuff) on the main objective or places that lead to it(usually around the corner in a hallway). Right click to blow up the stickies. When there is a spy around you should hit people with your bottle until you kill the spy. Use your grenade launcher and fire it into large crowds for 100% effective kills. Try to hit people with the grenades aswell for easy kills.
As a demo on offense you should use your grenade launcher to take out sg's(sentry gun's), large groups of people, and idle snipers. Grenades are all bouncy so if you can hit some thing around a corner, try bouncing them around it. When you run into a sentry that is hard to hit with your grenade launcher try using your sticky gun and fire stickies at the sg from a safe place, then detonate them and have the bottle at and press the G key at the same time :D.
As a pyro you should always be alert and spy checking, to spy check you randomly spray your teammates with your flame thrower, if some one catches on fire then they are a spy.
Also you should hide and when some one comes into your territory you should burn them for 4 second then hit them with your axe or shoot them with your shotgun. Never run out in the open as pyro, you will end up dieing. If you run into a heavy + medic combo then you should kill the medic first then try to kill the heavy, never attack the heavy first. I will expain that later.
As a Engy/engie(Engineer) on Defense your job is to defend important objectives bye building a turret(sentry gun), however you need metal to build one, you get 200 metal when you spawn, but if runs out when you build stuff or repair things. You can get metal by heading your cabinet in the locker, or by picking up ammo crates and dropped weapons, you also get ammo from these things. The Despenser gives you health and ammo, you can also build teleporters. Also have your shotgun ready incase you run into spies or demos. you have a pistol ,but thats for spychecking or sticky removal, you also want to put sentries in hard to hit places.
As a Engy on offense your job is to get your teammates to the battle faster, build teleporters to a safe, but close to their base spot. You want to have a fully upgraded sg and dispenser at your front line base.
As a sniper offense/defense your job is to pick off enemies who will be the hardest to kill, aim for the heads when your scope is fully charged.
As medic you heal people, but also to help push your team to victory. How do you do this? Simply find a heavy and start healing him, you will notice a uber charge meter going up, when its fully charged you can active it and make you and the heavy become invunerable for a few seconds, deploy it when you reach a critical moment.
As a heavy you should stay in small or critical areas and defend or push the your team forward. Use your shotgun for defending your self.
As a spy you should disguise yourself and mess with the other team any way possible.
The scout is difficult to dicuss, ask some else about this.
As mentioned before when you attack the medic first part. If your being healed and getting hit at the same time you are building up the medics uber charge faster.
If you want to play on a server with a bunch of blocklanders, simply go to these servers: Jcinks server and super toxics server.