Here's my tuppence worth;
If you are in a light tank, don't hide in the bushes next to the TDs, scout in front of them about 50 to a 100m so they can shoot the stuff you reveal.
For all types, always have an exit route planned.
Don't spam the map marker, and if you do ping it put in chat what's there.
Don't spam "help me" without qualifying what you need help for.
Don't spam "help me" if you are in a light facing down their entire team.
When you die, let your team mates know what killed you if you are not in range of someone else, especially if it's a hidden TD. Saves next guy rolling into that JPanther's sights.
Protect your artillery, I'd quite happily suicide a light if it means taking out their Hummel. If you see lights heading towards your arty, stop them however you can. [Ramming is fun and a perfectly acceptable tactic for a light to take out SPGs, don't listen to the haters.
Pressing any ammo button twice will reload your gun to that type, stop firing into the ground to reload, it's annoying to people who know what to do.
DON'T PLAY A STANDARD GAME IN A TIER 1 TANK!!!! You are hampering your team from the start as you are taking up a space that could be filled by someone else in a useful unit. You will also almost always get TK'ed within 3 nano seconds of the match starting because it really pisses people off. You'll be playing with the big boys soon enough don't worry. Also anyone who spams chat with "HAI we win now wif loltractor!!!11" is a friendless gimp who will never get to even kiss a woman other than their sister.