A couple things before you join the party.
1. Play either Heavy, Medic, or Engi to start off, they're pretty easy to learn the basics of, and are a huge help to your team. Since you're from DoD, Sniper might also be a good choice.
1.a) As a heavy, when you unlock the Sandvich, you can right click to throw it, which acts as a healthpack for anyone to use.
1.b) As a medic, heal everyone. Don't just heal one person. Make sure everyone around you is full health.
1.c) As an engi, upgrade everything. A sentry will destroy everything it sees, dispenser is a huge help to the team, and teleporters get your team to the fight quicker.
2. In the menu, click Advanced Options and turn on anything you like, but make sure you turn on "Play a hitsound when you deal damage", and "Display damage dealt over enemy's head" or whatever they say. The default hitsound is kinda annoying imo, but there are plenty of tutorials on the internet if you wish to change it.
3. You find items by playing. You might see players with all kinds of neat weapons, but don't worry, you'll have them eventually.
4.
http://wiki.teamfortress.com/wiki/Main_Page