If the water pressure in your shower is very low, it may be that you simply need to clean the showerhead. First soak it in vinegar. To do this, place vinegar in a plastic bag and use a rubber band to fix this around the showerhead, making sure that the showerhead is immersed in the vinegar. Leave it overnight.
If the problem persists, unscrew the showerhead and take it apart. Rinse it, scrub it with an old toothbrush dipped in vinegar and use a pin or a toothpick to poke out mineral deposits. Soak the parts overnight in vinegar, rinse them and reassemble them before reattaching the showerhead.