A plumbing system should have a main shutoff valve that controls water to the whole house and intermediate shutoff valves that control water for various areas of the house. There also should be shutoff valves on the incoming cold supply line for a water heater and stop valves (also called fixture shutoffs) that control water leading to individual faucets and appliances.
If an old shutoff valve -- usually a gate or globe type -- leaks at the packing nut, make sure it's all the way open or closed. If it still leaks, try tightening the packing nut with pliers or an adjustable wrench. (Don't crank down too hard, or you could crack the nut.) If it still leaks or if it fails to completely shut off water, repair or replace it.
Stop valves are often cheaply made and may fail to shut off water completely or may leak from the packing nut. Replace a faulty stop valve with a ball-type model, which costs a little more but is very reliable.
A globe valve inhibits water flow even if it is in good working order. Replacing it with a ball valve may increase water pressure.
An hour or two to repair or replace a shutoff valve
Groove-joint pliers, pipe wrenches, wire brush, flux brush, propane torch, fiber shield, adjustable wrench
Working with supply pipe
Shut off water upstream from the valve.
Repair parts or a new valve, pipe-thread tape, flux, solder