There are basically three possible reasons that your shower head has a low water flow. It could be that your home's water pressure is simply low. If your garden hose has a very low flow of water, then it is probable that your water pressure simply is too low for a good, strong shower. However, there are things you can do to your shower head to increase the flow of water regardless of your local water pressure.
Place a cloth around the nut where your shower head connects to the wall pipe or your hand-held line and then use a crescent wrench to unscrew the nut (the cloth is to keep the nut from being scratched by the crescent wrench).
Inspect the now-visible inlet to the shower head or your hand held shower head. Many models have a screen. Use your screwdriver to pry the screen out, being careful not to damage it. Hold the screen under running water until all dirt, sediment and other debris is gone. In some models you will need to use your screwdriver to carefully pry out a rubber gasket first, before the screen will come out. In some shower heads there is no screen at all.
Do not replace the screen yet. Instead, insert your screw driver into the inlet of the shower head and hook the corner of the blade on the star-shaped piece of plastic that is under the screen. Twist the screwdriver gently to remove this piece of plastic. Under this star-shaped piece of plastic will be another piece of plastic with one or two small holes in it. Tap the shower head on the palm of your hand until this piece of plastic falls out. You have now removed the component that restricts your shower head's flow.
Insert the screen just as you found it. Replace the rubber gasket. Screw the shower head back to the wall pipe or back onto the hand-held hose. Remember to cover the nut that you need to tighten with a cloth before you put your crescent wrench on it to keep the finish from being damaged.
Test your shower head. Even if your system has a low water flow the amount of water coming out of your shower head should be two or three times as great as it was before you removed the water controller.
Before screwing your shower head back onto the wall pipe or the hand held hose you may wish to wrap the threads with Teflon tape, also called plumber's tape. This will help to keep the connection from leaking.
Removing the flow restrictor or flow controller will result in added cost to your utility bill from increased water and power usage.