How do I extend landing gear?
Hitting g will toggle the landing gear. Additionally, 0.18 introduced a way to do this with mouse. Next to the altimeter is a set of buttons for lights, landing gear, and brake lock.
Besides MBraedley's answer, which covers 90+% of the use cases, you can right-click on each landing strut to toggle it individually. This can sometimes be used to right a crashed lander enough to blast off of the surface.
Landing wheels work on the same system.