It's really important that you get a good feel as to how fast your new group likes to run through trash - the other tank should be able to help you there. I personally, like it when the next pack is pulled whenever the current pack is almost dead. This gives the tank pulling the new pack the time to position them if needed.
Ask your other tank for specific fight tactics before the boss fights - he should be able to tell you whatever details you need to know quickly, and it'll prevent you from screwing up as well.
Tank communication is really important - if you approach the other tank with an attitude of "I'm a good tank, but I realize I'm the new guy, so could you please share your wisdom" he should be able and willing to help you.