Use your mic!

If you only use your mic to say "ugh, this team sucks" communicate more, don't think that the rest of the team should know what you're thinking.

When playing in randoms there is a variety of player some more experienced some less.

Teams that communicate win more often than not.

You don't need to say "they're taking A" the game already says that, but if you go there and see three guys or just one, it helps to say "three on A" then the rest of the team knows what they are up against when going there.