Gravity should allow the water in your pipes to run out the open faucet. You can repeat this procedure for both hot and cold systems. Your hot water heater will also have plenty of water inside it. You can access this water from the valve on the bottom. Again, you may need to open a faucet somewhere else in the house to ensure a smooth flow of water.
Sediment often collects in the bottom of a hot water heater.
While a good maintenance program can prevent this, it should not be dangerous. Simply allow any stirred up dirt to again drift to the bottom. There are certain climates and geographic locations where finding water will either be extremely easy or nearly impossible. You'll have to take your location into account when you read the following. Buy a guide book tailored for your location, be it desert, jungle, arctic or temperate.