You should select the role (tank/dps/healer) you wish to be in before queuing for a dungeon in the dungeon finder.
It’s found on the left side of the Dungeon Finder interface
If you’re at least CP160, properly geared up and can handle Public Dungeon bosses on your own without dying, you can queue as a Tank for normal non-DLC dungeons.
Just be sure to slot Inner Fire to get the boss to attack you to save your other mates from dying and slot some heals/resistance abilities if you can.
If you are in queue for more than 5 minutes in the Dungeon Finder, you should requeue again (leave queue and join queue). Sometimes you may get stuck and this will fix it.