Open the shower door fully. Place a level on the shower wall where the door closes. If the wall is out of plumb, the problem is with the framing of the shower, and you will need a professional to take out tiles and shim out the frame.
Stand inside the shower and close the shower door. Loosen the hinge screws on the door with a Phillips screwdriver. Only turn each screw two revolutions.
Ask your helper to place 3/8-inch shims between the bottom of the door and the shower base. The hinges will readjust as you slide the shims under the door.
Retighten all of the hinge screws. Remove the shims and operate the door. If necessary, loosen the screws on the door catch and move it up or down as necessary before retightening the screws.