You really need to find our what your water pressure is before you decide to regulate it. You can obtain a pressure meter from a plumbing supply store (rent it). Attach it to an exterior hose bib and turn on the water. Your water pressure should be between 50 and 70 psi. If it is greater, you need a pressure reducer on the main supply line. This could be installed at the meter or just inside your internal shutoff valve.
Excess flow from a shower is easiest to solve by buying a low-flow shower head replacement. Regardless of pressure the the shower will dispense water at a fixed rate (2.5 gpm is common). If your only problem is at the shower, control that with a new showerhead and keep the pressure for washing your car and watering the lawn. If pressure is excessive throughout the house, reduce it to avoid damage and possible pipe and fixture failures.
