*Understand the goal of your gametype and how it may impact the gameplay you experience
Keep it simple for yourself. If you're playing freezetag, for example, chances are if you know a teammate is frozen at Room X on the map at least one of the enemies knows this too. This enemy is going to most likely expect someone to attempt a thaw. Be prepared to be fragged or at the very least shot at as you get close. If you're repeatedly getting shot under such circumstances you really need to evaluate your approach and make some much needed adjustments. You need to understand the opposition's goal is to freeze you and your teammates (and keep you that way! :~p). Don't expect to walk up casually and thaw your mates.
*Learn the Maps
It sounds very obvious but it's amazing how many people you will come across that simply seem to lack general knowledge of map layouts. This is crucial. Load up some practice games and make note of advantageous areas of maps while you sharpen up your aim. Also make mental notes regarding what path is faster from Point A to Point B; extra helpful in the event you are playing CTF
*Don't get mad that someone keeps killing you
Keep in mind your teammates are most likely equally annoyed with you for dying frequently. Some gametypes this can be a bigger deal than others; CA for example.
Raging in console and calling Player X various names just makes you look silly. Similar to the first point; take a second to evaluate what you're doing when this is happening and try to make adjustments. Even if you get similar results at first you need to keep trying different methods in hopes of discovering one that will work for you.
*Be a team player
Most likely if you're newer to the Quake Universe you may be a liability for your team to some degree at first. This is to be expected. Your first few days/weeks learning the game is a great time to emphasize a skill that will make you an asset to your team even as you learn the ropes. The BUDDY SYSTEM :~)
Find a teammate and try to stay nearby. You'll either draw enemy fire allowing your teammate a good opportunity at an easy frag or you'll benefit from the opposite scenario while your teammate is taking fire.
It's a win/win for you and your team.
My apologies the list is fairly short, but these are VERY general things I seem to see a lot of when I have time to play. Hopefully it helps someone.