This varies dramatically between which type of Feast game mode you are queuing for. Personally, I would start with 8 versus 8 casual.That game mode is not ranked, so people take it far less seriously. Further, the teams are bigger, so even if you're making mistakes it is less likely that anyone will notice. You have seven other teammates to hide behind in that game mode. It will seem far more chaotic that 4 versus 4, but it's just for fun. It's a friendly environment to learn in, and you'll still be getting tomes/wolf marks. If queues are still active for this game mode, then I would try to start with that.
4 versus 4 solo-queue is a different story. People tend to take it quite seriously (often too seriously) because that game mode is ranked and heavily dependent on team composition and coordination. Teams are random (as they should be), so it's a roll of the dice as to how good your team will be when you get into a match; however, having even one bad apple can completely toss the match. If the match happens to be a qualifier for someone, then they will be very unhappy to see a new player on their team because qualifier matches are the only way to advance a rank. Qualifiers happen when you get to your point cap for the next rank. If you win the majority, you go up a rank. If you lose, then you get shafted back down and basically have to start over (hence the rage when people get noobs on their team during qualifiers).
That said, I still most definitely recommend getting into 4 versus 4 as soon as possible. Firstly, the queues are faster. Secondly, that game mode is more intense and requires more attention and skill to play well. If you are interested in a more competitive setting, then 4 versus 4 is it, because everything you do will effect the team and outcome of the match. You can definitely jump in right away, but you'll want to brush up on a few things before you go in.
Firstly, get to know your role. Tanks do not function anywhere near the same in PvP as they do in PvE. You have ZERO aggro. Your job is to: Harass the enemy healer (cause as many casting interruptions as possible), defend your healer (save their butts from being interrupted or worse, killed), and support your dps (Full Swing is gold for this, use it wisely). It's one of the hardest jobs in PvP, because you have a lot to juggle. It takes practice, do don't get discouraged. The next thing is to get to know your PvP actions. Read the tookits, and figure out when and where it is the best time to use them. Some of that comes with experience, but you can also theory some of it out just by reading them. AT THE VERY LEAST, they should be put on a hotbar so you can use them. After that, it's a matter of knowing how the game mode functions and what your priorities are (who to attack and when, if you should be on the offense or defending your healer, if you should chase for a kill or fall back, etc). Most of that comes with experience, so you'll just have to play and be baptized by fire.
If you have any specific questions, then ask them here and I'm sure I or someone else will be happy to provide you with some advice. Take it with grains of salt, though, because there is no one-size-fits-all answer for PvP. It's the nature of the game. Otherwise, have fun and good luck out there.