If you have a faucet beside or close to the main valve, turn it on and see what the pressure is like there. If the pressure is strong, but weak elsewhere in the house, then the problem lies with the piping inside your house. You will have to investigate, but I gather that it is probably a case of a slow blockage somewhere within the piping system. If the pressure is weak right beside the main valve, then the problem lies with the city piping. If this is the case, then you should contact the city and tell them. Another possibility is that the main valve itself is the problem. Try turning the valve off completely and then turning it on again. Sometimes the main valve needs to be tinkered with in order to have full water pressure.