Pull on the spark plug wire to remove it from the plug and then turn the plug counter-clockwise. Once the plug is removed the engine will not start so you don't need to worry about accidentally cutting yourself. Putting in the new plug will be the last thing you do. Next drain the gas from the mower if you didn't already do that last fall. Just tip the mower over and pour the unused gas into a coffee can or other container.
Gas goes 'stale' after a month or two and can plug up the engine.
Do not pour it onto your lawn or driveway. Once the job is through you'll take the gas to a recycling center. Next we'll sharpen the blade. Turn the mower over and notice the direction of the cutting edges of the blade. Then remove the blade.