Hello, i hope this guide will help you learn something!

First of all, you need to play as an Engineer class. This guide only works for Engineer!
After you joined into the game, choose Engineer or simply press number 6 on your keyboard. Walk out from spawning room, because you can't use construction PDA as Engineer in there. After you walked out of spawning room you need to search for the perfect spot to use your PDA and when you find it, press number 4 on your keyboard and then the panel with 4 buildings will be shown for you.
The first building is "Sentry" the name speaks for itself. It's targets the enemy team's members
The second building is "Dispenser". It's heals you and your team, also it can give metal for you and friendly Engineers and let's mention the fact that it can give ammo for your team.
The last two buildings called "Teleporter", there's two types of them, Entrance and Exit. Entrance teleporter won't work without the Exit teleporter and it's works with exit teleporter. If you put both teleporters then if you or your team member will stand on Entrance teleporter then it will teleport to the Exit teleporter
You can also rotate buildings and pick up them when they're already built by pressing Mouse2.
Also to mention, if you use gunslinger then your sentry will turn into mini-sentry

Now go boyo! Show the world that you're an nerd