There are two types of tub drain stoppers: lift & turn and toe tap.

Lift & Turn

The lift & turn stopper for the bathtub unscrews counterclockwise while in the "full up" position.

Toe Tap

The toe tap stopper also unscrews counterclockwise while in the "full up" position.  However, the cap may unscrew from the plunger assembly before the entire stopper comes out. In that case, there is a nut that will be exposed that can be used to unscrew the plunger assembly from the drain.

If either of these is "spinning" without unscrewing, the threads are stripped, and you will need to exert upward pressure while unscrewing to uncouple this from the bath waste. If that doesn't work, you will have to unscrew the bath waste flange from the drain pipe.

This will require access under the tub.