This single question splits the problem in half. If only one tap is weak, the cause is at that fixture. If pressure is low everywhere, the problem is at the main, the regulator, or in the pipe running through the house. Check a few taps on both hot and cold before doing anything else.
Unscrew the aerator on the weak faucet and rinse the screen. In hard water areas across California this fills with white scale within a year or two. If the aerator is clean, the cartridge inside the faucet may be partly blocked, or the angle stop under the sink may not be fully open.
If cold pressure is fine and hot is weak everywhere, sediment in the tank or a clogged nipple at the heater outlet is the usual explanation. Flushing the tank often restores it. This is also an early sign that the heater needs attention before it fails outright.
Most California homes have a pressure reducing valve near the main shut-off. They typically last eight to fifteen years, and when they fail, pressure either drops away or spikes dangerously high. A gauge on an outdoor hose bib should read roughly 50 to 70 psi. Much below that points at the regulator.
In homes built before the 1960s, galvanized steel supply lines corrode from the inside. The pipe wall builds up scale until the opening is a fraction of its original diameter. Pressure that has fallen off gradually over years, plus brown water on first use, is the classic pattern. Repiping is the only real fix.
A significant leak between the meter and the house steals pressure before it reaches your fixtures. Shut every tap, then watch the water meter. If it still moves, you have a leak that needs locating. This test takes two minutes and is worth doing before anything else is replaced.
Occasionally the pressure drop is upstream of your property entirely, especially after street work. Ask a neighbor whether their pressure changed at the same time. If it did, the utility is the right first call.
A real plumber, a flat-rate quote, any hour of the day.