Here are some random advice for beginners. Perhaps Swiss can copy/paste some of the in to the initial post? Please add more advice so we can have them in one place - they are needed...
Never go in to a fight outnumbered if you want to win it. Only very experienced player can do this with some hope of success (kamikaze attacks are ok if you are trying to delay the other team).
Make sure you have at least one skill that interrupts. This is VERY important in this game. Then you can interrupt someone trying to take over an Outpost (op for short), interrupt an opponents skill, interrupt a relic steal etc.
The ops are of major importance. If you have them – keep them safe and interrupt anyone trying to take it. If you don't have them – take them or help your teammates taking them.
Keep your team's Templar alive. Interrupt/kill/taunt opponents trying to kill your templar. That way the entire team will have better chances surviving.
Kill the other teams Templar.
Stay close to experienced players – they know where to go and when. Don't go solo unless you know what you are doing.
There are regeneretion spots on some of the maps. Don't use them (unless you want to frag) – you'll only lose time and you don't help your team mates standing there waiting...
Stop/delay/kill opponent carrying relics.
Pick up relics and carry them to your base. But note that most often it is wise to kill the opponents around the relic first, since the relic will harm or hinder you, and then pick it up.
Killing is easy, but you hardly get any rating for it unless you win the game. So winning the game is primary, killing secondary, if you want to get high rating in a game.

/ Pimp