Training

The teleporter can help teammates by helping them get into battle faster,
if a spy is sapping your teleporter DO NOT stand on it while fixxing it.
According to the laws of TF2 the spy can teleport and kill you,
if a spy stands on a teleporter and an enemey teleports at the same time the spy will die.
Now finally the dispenser, the dispenser can replace a medic and can give ammo which any other class can't do and only the engi can use this skill.
Building the dispenser next to your sentry is the most common idea but you don't have to it's just a suggestion to do so.

The sentry gun is a fighting machine.
its used for distractions to help you or another get away if your up to give the metal,
a mini sentry is not terrible it's a fast way to escape a fight while running for the intelligence.
the mini sentry can be used to distract the opposing team from doing any physical harm to anything,
The mini sentry is not as crappy as you think.
This is the end of lesson 1 and i hope you learned something.