Here's something for you fellas:
The Complete Guide to Team Fortress 2
Besides going over the game and the play mechanics in detail, it has seperate links explaining every class. I hope it helps! You can also ask me any question. I'm sure I know pretty much how everything works as I can use any class now pretty well, depending on what the best situation is. General tip for learning the game... choose Soldier or Pyro first. They are VERY straight forward, easy to use, and are useful almost anywhere. Just point and shoot anyone you see as you learn the goals (explained in the optional video when first enetering a map) and map layout.
I also suggest learning exclusively on the map 2Fort. The other maps are mostly one big, massive tug of war at choke points, but 2Fort is a bit less centralized. You can roam around a bit more and get involved in mini-skirmishes to get more focused practice. The goal of 2Fort is two things... (1) Stay around/inside your fort to guard the intelligence suitcase in the basement, (2) go to the enemy fort, enter the basement and take the intel. You get 1 team point if their intel is brought back to your basement.
If someone dies holding the intel, it sits on the ground for 60 seconds. You can not touch your own intel (ever) so the defense has to stand guard around their fallen intel for the 60 seconds so it can automatically return to their base. The offensive team needs to continue to try to scoop up the fallen intel and take it to their intel rrom for the point. If a team gets three intel capture points, they win the round. The match sometimes ends once on team is gets 2 or 3 round points, or on a timer. Some 24/7 2Fort servers simply last forever.
ScottyL: 75/200 means that in your "inventory" you are holding 75 pieces of metal, but if you get more you can hold a maximum of 200 pieces. If the upgrade process stopped for your Sentry, then I'd say you built it to Level-3 already.
The Sentry (and all of your builds) has a horizontal bar when you make them that shows how much metal it has and and much more you need to add to get it to the next power level (with your wrench banging on it). There are three levels, and once you reach L3 It can't be built on anymore. You can however repair it with your metal reserves if it gets damaged.