All of the best looking starships have landing gear that deploys when the ship lands, right? You can make landing gear for your ship by making a downwards-facing cannon: place a piston facing the ground, and then on top of that, place a Mechanical Block. You can have as many landing gear legs on your ship as you want. They have no function beyond looking cool (Edit: While landing with landing gears, your ship will not be able to get less than one block off the ground. This makes it useful for boarding ramps so you don't land on the belly and break the ramp when you deploy it.), but they may help you win starship building contests. Landing gear legs do not extend more than one block towards the ground, so build carefully.
Landing gear deploys automatically when a ship is released, and retracts automatically when the ship is piloted. After you build your landing gear, pilot the ship and then release it. If built properly, when you release your ship the landing gear will extend. Be aware that every extended landing gear counts as an additional block to your ship, due to the piston head technically being a block (technically.) This might prevent you from taking off again if your ship's block limit is exceeded.