Have 10 year old gas water heater very low water pressure throughout the house:confused:

Your water heater and your water pressure are not related unless you only have low hot water pressue, do you? Lets start by flushing, not draining, your heater. Leave the main on and open the drain spigot at the bottom of the tank. You may need to add a hose to direct the water to a drain. Flush for about 5 minutes or until the water is totally clear of color or debris.