Just tell 'em it's your first time, assume the OT position and watch what everyone else is doing. Usually the other tank will give you the quick and dirty per boss so you're not taking up a lot of time and you get the gist of where you need to be.
I ran into CT n00b as Hell and figured it was like every other dungeon out there... If you're not main, you grab adds. Turned out I was basically right. I learned little nuances with each run, but for starting out, I just felt my way through. Research and videos help but none of that compares to actual experience. There's no dps race in here so take your time, watch and learn and don't be afraid to ask questions