There are only a couple ways to get air in the water line, coming from the well. The most common is a hole, or leak in a fitting, or the suction line if it is an above ground pump. if it is a...
Incorrect installation is a common problem. Other factors include switching off the mains and switching them back on again while keeping the taps closed. You may also have a faulty vacuum breaker
It depends on your system. If your on city water you should be able to pull off the airaters on the sinks and open up all your fixtures until the air is out. If your on a well you could try the same...