Causes of low Water Pressure
Showerhead
The showerhead has a screen inside that can get clogged by mineral deposit build-up.

Solution: Remove the showerhead and then remove the screen. Using a small toothbrush, clean the mineral deposits from the screen or use a cleaner specifically for removing mineral deposits.
Sink
Mineral deposit build-up can clog the aerator in a sink.
Solution: Remove the aerator by unscrewing it from the faucet. Using a small toothbrush, clean the mineral deposits from the screen or use a cleaner specifically for removing mineral deposits.
Shut-off Valve
Mineral deposit build-up can clog a shut-off valve.
Solution: Unscrew the stem from the valve, then clean out the valve using a small toothbrush.