In MvM press tab to view the leaderboard. Directly do the right of your name is your tour count. If there is no number there, you have zero tours. The number increases with every four missions you complete in Mann Up.
People groan and complain when zero tourers enter the server because more often than not they're huntsman using gibustards and other morons who don't know jack shit. As someone with fifty tours, it gets pretty damn tiring. If you don't know how to play MvM, please read a guide and learn the basic meta (I know, I know, boooo. Let me kill robots the way I want to, boo. Just do it for now to learn the basics and when you have more experience you can mess around more and play as Spy or Sniper or Pyro and people will trust you). We're not mean, just tired of people who refuse to learn the difference between MvM and your average 2fort game or whatever.
Let me give you some tips:
Nobody likes having more than one of a class that's already taken except for Bavarian Botbash in which 4 soldiers, 1 scout and 1 engineer is the best strategy, and on the last wave of Hamlet Hostility in which all six players change to phlogistinator Pyro.
Wait for the demoman/spy/sniper to take out smaller medic-bots before shooting them or they will "pop" their übercharge.
As heavy, you will never ever need ammo upgrades on your minigun, the engineer's dispenser should be enough.
As engineer, full dispenser range upgrades are needed for helping Heavy and Soldier maintain a constant damage rate.
As scout, avoid attacking robots and priorities collecting cash.
As demoman, you will never need your grenade launcher so I do not recommend upgrading it.
As Soldier, upgrading clip size means longer reload times. Prioritize upgrading reload and firing speed.
Don't throw mad milk or jarate at the sentry buster. Unless you're the Engineer or it's right next to you about to detonate, completely ignore it. If you're the Engineer, use a Rescue Ranger to pull your sentry away to safety when it's about to detonate. If you don't have a rescue ranger, take your sentry and run like hell.
Listen and take advice from higher-tour players. They know their shit, they've done the same levels hundreds of times, usually literally. If they have any pointers or instructions, listen to them.
I hope these tips help. Good luck manning up!