Well now, I was looking through the site and found out that there are very few, if any, How-To's on Team Fortress 2, one of the best recently released games made by Valve.
So I decided to instruct you on how to do something fabulous in the game.
How to build an unstoppable gun as an engineer.

Some things you need:
-Wrench------------------- Standard Engineer Melee weapon, join the engineer class to obtain.
-More than 600 Metal-- This can easily be obtained, despite the fact you can only carry 200 at a time.
-A good spot------------- To place your Sentry of course!
-Time and Dedication- This is not an easy task, patience will be rewarded in the end.
-Goggles------------------ They do nothing.
Optional equipment:
dispenser----------------- An easy way of obtaining metal.
Shotgun------------------- To help defend your sentry when it is malfunctioned.
Pyro------------------------- Very good spy checkers.

All of this may appear confusing, but everything will be revealed in time.

Step 1: Choosing a Server, Team, and Class

First, we need to find a map to build our sentry, or else we cannot build one of level 3.
ctf_2fort.bsp is a good map for beginner engineers, so i highly suggest that.
After you find a server of your choice, you will be prompted which team to choose. This does not matter, as both team RED and team BLU can build sentries, given the player is the right class. Which leads onto our next substep.
You will be asked which class to choose. Select engineer, or some other class, but that wouldn't allow us to build a sentry. If at any time you get tired of your current class and decide to build your level 3 sentry, press the ",", "<", or comma key on your keyboard located next to your ".",">", or period key.
Select engineer, so we can start building.

Step 2: Beginning the Building

No, we can't build a level 3 sentry just yet.
We must find a spot to build it first!
Anywhere is fine, however there are spots to avoid.
A rule of thumb as an engineer is to build a sentry where it can oppress a choke point, and be hard to hit without being hit itself.
Choke points are positions in which the enemy is often found going through, such as a narrow hall to the intelligence room.
This does not mean build a sentry in the hall, but rather gaurding the exit.
It may take some time finding a fit position, but after trial and error you should realize a good spot.
So now we have a spot to build our level 3 sentry, I chose on the chair in the intel room.
But wait! We still can't build our level 3 sentry!
In order to build a level 3 sentry, we must first upgrade our level 2 sentry. But where do we get a level 2 sentry?!
From upgrading our level 1 sentry of course!
Scroll to select your building numpad, located under the wrench. Or hit "4" (four) on your keyboard.
A lot of options to choose from, but just hit "1" (one) for now.
So now there is a ghost of a sentry floating on the ground. Walk around to your position to build it, and use the left mouseclick to place it.
Right click rotates the ghost.
Now you must wait until it erects.

Step 3: Upgrading to Level 2 and Level 3.

This is probably one of the most rigorous steps in building sentries; upgrading them.
It takes a lot of metal, and patience.
Start hitting your sentry with your wrench, you will notice your metal level goes down with each hit.
Once that number hits zero, you can no longer upgrade your sentry, and must find some metal.

Metal can be found in a variety of places:
blown up building parts.
Dispensers.
Old weapons of dead friends/foes.
Ammunition packs scattered throughout the level
Resupply rooms.

I suggest going to the nearest resupply room to get your metal, this guarantees you will get 200 metal.
I also suggest building a dispenser before upgrading your sentry so you don't have to walk far for metal.

Once you get more metal, continue hitting your sentry with your wrench until it upgrades to level 2 and even level 3.
Congratulations! a level 3 sentry!
But we are not finished yet...

Step 4: Maintanence and Conclusion

Now you must watch over your level 3 sentry carefully, as it is now a target for spies to sap.
If a spy saps your sentry (an icon appears in the top left corner of the screen, and your character will yell out "SPY SAPPIN MAH SENTRY!"), simply wack the sentry (and spy if noone else kills it), until the sapper breaks.
This can also cause your level 3 sentry to lose health, which can be repaired by hitting it with your wrench, provided that you have enough metal.
When your level 3 sentry fires its bullets or rockets at enemies, you must also resupply it with shells and rockets, by hitting it with your wrench, if you have enough metal.
Your sentry will stay there until it dies, which is usually caused when someone kills you then is able to take out the sentry.
Pyros can make great spy-checkers, as they can flame the area and reveal enemies.
Medics may also be found healing you when you repair a sentry that is under fire, as they can earn an acheivement for doing so.
And as another side note, if you see a friendly engineer's buildings damaged, repair them, because they can use all the assistance they can get.

Keep on building level 3 sentries, as it is a vital part of your team, just as a medic is, or a demoman.
Everyone is counting on you, and the other engineers, so stay alert.
CONCLUSION GOES HERE.