If you experience low water pressure throughout the entire residence and it occurred suddenly, you should check the valves located on each side of the water meter and verify they are completely opened. These valves may be partially closed by children playing, or if a contractor was working on the system.
If the low pressure has occurred over time and is noticeable at only one or two faucets, try cleaning out the aerators on the faucets in question. Simply unscrew the aerator from the faucet, remove the pieces, and rinse water backwards through the aerator.