First, narrow it down
Run the cold water in the kitchen, then a bathroom sink, then the tub. If only one fixture is weak, the problem is at that faucet. If everything is weak, the cause is further back: the main line, a pressure regulator, or, if you're on a well, the pump and pressure tank.
Also check whether it's weak on hot only, cold only, or both. Hot-only weakness at every fixture points at the water heater or its shutoff, not the faucets.
If it's just one faucet
Clogged aerator (start here)
The aerator is the small screen at the tip of the spout. It catches grit, rust flakes and mineral scale, and it's the cause more often than anything else. You can unscrew most aerators by hand or with tape-wrapped pliers, rinse the screen, soak it in white vinegar if there's white buildup, and put it back. If the flow jumps back to normal, you're done.
Debris inside the cartridge
If cleaning the aerator doesn't help, grit may be stuck in the cartridge. This often shows up right after work on the water line, when sediment gets stirred up.
A partly closed shutoff valve
Look under the sink. A valve that isn't fully open, or an old valve that's partly seized, restricts the flow.
A kinked or failing supply line
Easy to miss, especially if the cabinet is packed and something has pressed against the line.
A clogged pull-down spray head
In kitchens, the spray head itself has small holes that scale over. More on that in kitchen faucet problems.
If the whole house is weak
On city water, a pressure regulator that's failing or a partly closed main valve can drop pressure everywhere. If you're on a private well, which is common on the rural roads around Sultan, the pump's pressure switch, the tank's air charge, or a clogged sediment filter are the usual suspects. Whole-house pressure problems aren't a faucet repair, and a good pro will tell you that instead of replacing parts that aren't the issue.
Well water and mineral buildup
Homes outside city limits often see sediment in the lines. If you're cleaning aerators every few months, ask about a sediment filter. City of Sultan water comes mostly from Lake 16 and is treated before it reaches you, so heavy scale is less common in town.
What a repair usually involves
Cleaning or replacing an aerator takes minutes. Replacing a cartridge is a short job with the right part. Replacing a shutoff valve or a supply line takes longer because the water has to be turned off further back. See faucet repair for what else is checked at the same time.