Like one big post at the beginning already said, play a Tank.

Start the tank from level 1 and do dungeons to level it.
Communicate with your group at the beginning and tell then that you are a new player and that it is your first time tanking the dungeon. The big majority will understand that and react positive towards you.

Some will tell you to take it easy and will try to help you and give advice.
Others will push you into the cold water and tell you to pull everything. (do it, laugh in case of a wipe and try again with less)
And again other will say nothing, so take your own speed.
But the most important thing is, take the friendly advice and critique to improve your tanking, they say that to you to help you.