I've been playing L4D since the first game came out, for 15 years. I ran through 2 CDs of L4D2 on Xbox 360 and gave my console red ring of death because of it. I've also made friends from random lobbies in this game on Steam, so I know how to be a team player.

I've seen a few guides like this on here but they're either too specific or too targeted for new players.

(yes, with random strangers)

1.) Do NOT stay far ahead or away from your team. Stick together OR opt for single player with a bot team.
2.) Be mindful of friendly fire. You may get killed or even kicked by teammates if you damage them too much, even unintentionally.

1.) Hunters, Jockeys, and Smokers can be pushed off players. Push them enough times and they will die. This is often more effective than shooting close-range, whereby you risk injuring your teammate.
2.) If someone is lagging behind, special infected can pin them in places they can't be rescued from if they go down. Keep an eye on those behind you.

1.) SHOTGUNS should not be aimed at your teammates, ever. The edge of the cross-hair should not cross a teammate's outline.

1.) If you aren't comfortable typing, the in-game voice lines are very useful (x).
2.) Hover over an item and press (z) to shout it out to your team.
3.) You can also use your flashlight (f) to indicate items, by flashing it repeatedly.

1.) If someone is being super toxic, you can vote to kick them in start menu (don't expect this to work every time).
2.) The easiest way to deal with people spamming abusive nonsense in chat is to ignore them. If what they're saying bothers you a lot, please leave and find another lobby. Trash people like this will only get satisfaction out of you being upset.

Hope this helps. Happy surviving!