If you have a leak where bathtub meets the wall the best thing to use is generally a silicone substance to fill in the area. I would use a silicone caulk to ensure that the leak is both sealed and will stay sealed if movement occurs during use of the bathtub.
It all depends upon if you are talking about a leak when it rains or if you have a leak with constant water seepage. If you have a leak in the water source pipe you will have to remove the section of wall and do your repairs and replace the walling.
Is the leak near or around the faucet to the bathtub? If so, you may need to replace the seals around this particular water fixture. Call a plumber if you are unsure as you can grow mold and mildew.
