shower shock,when hot or cold water is used in other parts of house
Pressure
Go to the local hardware or home improvement store, and get a water pressure gauge. Most likely they'll have a simple gauge that can attach to a garden hose spigot.

This will allow you to measure the water pressure in your home. You'll want to make sure the pressure is somewhere between 40 - 80 psi.
If the pressure is low, you'll want to look for a pressure reducing valve (PRV)

If you have one, you could try adjusting it to raise the pressure. To adjust the valve:

Loosen the locking nut on the valve (The nut just at the top of the bell on the valve), and back it off a bit.
Use a wrench to adjust the bolt on top of the valve. Clockwise will increase the pressure, while anticlockwise will reduce it.
Once you're finished with the adjustment, snug the locking nut back down (not too tight).

If you're adjusting the valve, and the pressure still doesn't seem to be going up. You're either not getting good pressure supplied to the building, or the PRV is busted. If you're on well water, you'll have to start working towards the well to figure out why the pressure is low. If you're on city water, you should be able to contact the city to see what pressure they're supplying to the house.

Volume
Without being able to inspect the plumbing, this is a difficult problem to diagnose.  The first thing to check, is the size of the supply pipe. You'll want to have at least a 3/4&quot; supply, though 1&quot; is possible.
Next, you'll want to look for areas where this size is reduced.  The main trunk plumbing should be the size of the supply pipe, and should only be reduced where it splits off to supply a fixture.  If you have a reduction in size that's supplying more than one fixture, you may have found your problem.
If you find this to be the problem, re-plumbing parts or the entire system is likely the only solution.