This walk-through separates the water heater work a homeowner can do from the work that belongs with a licensed plumber. Read the label on your unit and your local rules before you start, because they set the limits for your tank.
What you can do. Turn off the power at the breaker, or set the gas control to pilot, following the label. Close the cold water supply valve above the heater. Attach a garden hose to the drain valve at the bottom and run it to a floor drain or outside. Open a hot tap elsewhere in the house so air can enter the tank. Open the drain valve and let the water run until it comes out clear. Close the drain, remove the hose, reopen the cold supply, and run the hot tap until the air clears before you restore power or gas.
Watch for scald and spill. Water inside the tank can be very hot. Let the tank cool before you drain it, and keep the hose clear of foot traffic.
What belongs with a plumber. Leave gas valve and burner work to a licensed professional. Leave the temperature and pressure relief valve to them as well. Leave anode rod replacement, any stuck or leaking drain valve, and any change to the pipes to them. If the drain valve will not close after you have used it, stop and call.
What to ask the plumber. Ask whether the unit is vented correctly, whether the relief valve discharge line is routed to a safe spot, and whether the anode rod has been checked. Ask how their work interacts with the tank warranty, and ask for the answer in writing.
Age as a planning input. Write the install year on the tag or in your record. Flushing does not change the age, but the age tells you when to plan. Keep the install year with your maintenance notes so the plumber can see what the unit is due for.