Unless you’re a centaur (half man, half horse), you’ll likely be getting off of your horse at some point. Here are the steps to take to dismount your horse according to Katherine Blocksdorf of About.com:

Starting off
Have the horse stand still and remove your feet from both of the stirrups.

Hold tight
Put both of the reins in your left hand with enough tension to stop the horse if it tries to move forward.

Lean forward and swing
With your hands just in front of the horse’s withers, move your weight towards your arms. After that, swing your right leg over the saddle and slightly push yourself away from the horse so you don’t go near the equipment or his legs.

Bend your knees and lead the horse
To help with the shock of hitting the ground with your feet, bend your knees as you land. Now that’s you’re off, you can safely lead the horse where you need it to go.

Source: About.com