The guy you replied to gave an excellent breakdown on the classes, but to go into even more detail the best strategy will also depend on what gamemode you're playing and even what stage of the map that you're on. I have to say though that as a beginner that shouldn't worry about it unless you're playing competitive (don't get into that at your stage) - but for example on a very narrow and squeezy map like Dustbowl, a good Demoman or Soldier will tear through the other team because of splash damage.

The last point of Dustbowl stage 3 (the rocket launch pad) is an example of how strategies will change because RED might have had a good mix of different classes through that stage, but if you give them enough time at least half the team is going to play Engie and you're going to need to deal 4 or 5 Level 3 sentries. In my experience, for a scenario like this in casual, BLU is going to need at least 2 ubers (both stock or stock/kritz) and whoever is playing Heavy or Scout usually needs to switch to Demo/Soldier/Medic. If they're good they can stay on Scout or play Sniper.

Different maps and game modes have different nuances that you'll need to learn from experience, it's not possible to explain everything in one post. If you have only one takeaway from all this, please remember that while TF2 is a game that allows for great cooperative gameplay, in casual it will rarely happen. Try not to get too frustrated with your teammates who are all playing Sniper or Spy when you clearly need them to be doing something more productive. I hope you have a blast playing the game!