Landing gear must be deployed before attempting to dock on a landing pad or land on an atmosphere-less terrestrial body with a ship. If the landing gear is not deployed when you are near the landing pad you will receive an audible warning from the ship's computer. Failure to deploy the landing gear before docking can damage, or even destroy, the ship.
There is an indicator on the right hand side of your ship HUD showing the status of the landing gear. The blue indicator shows that the landing gear is deployed (down). When there is no blue indicator the landing gear is retracted (up).
You are able to assign a button or key command to control the landing gear. This is done via the client control options. The default setting is the "L" key.
Deploying the landing gear in normal flight will decrease the speed of the ship. You cannot deploy the landing gear whilst in supercruise mode, or during hyperspace when travelling to other systems. If the landing gear is deployed when trying to enter supercruise mode, or a hyperspace jump, you will receive an alert which instructs you to retract (raise) the landing gear first. If you try to land on a planet with an atmosphere, your ship will burn and be destroyed because ships do not have heat shields.