If your bathtub drain is clogged and your bathtub has water in it you'll need to use a bucket or a pump to remove the standing water in the bathtub.
You can take a bucket and remove as much water as you can from the bathtub and then pour it down the toilet and then once you get most of the water out you can use a wet/dry shop vac to suck the rest of the water out of the bathtub.
Then pour some Drano Drain cleaner down the drain and let it sit and work.
Or you can pour some baking soda down the tub drain and then pour vinegar into the drain and cover it and let it sit for around 30 minutes.
Then pour some boiling hot water down the drain which should then clear the drain.
You can also go to Home Depot and buy a hand pump that will suck the standing water out of the bathtub so you can clear the drain.