If your water pressure is fine in the rest of your house, then the difference in water pressure is an issue with the shower head or controls.
TIP: Remove the shower head and check the water pressure of the shower. If the pressure is better, make sure to thoroughly clean your shower head and then screw the shower head back on and check the pressure once more. If this does not work, the shower head may need replacing.
If you have removed the shower head and the water pressure remains the same, there may be an issue with your shower controls and this will need to be looked at by a plumber.