How do I change ships?
I start in X3: Albion prelude as a Humble Merchant. It starts with a ship called Mercury and another called the Discoverer.
I have travelled to the shipyard in Argon Prime, but I don't have any idea how to sell my ships.
How can I use the Discoverer? I haven't found a way to switch to the Discoverer cockpit. Am I allowed only to give it follow commands?
A) In stations
You dock both ships inside a station and then you can switch over to a ship by selecting it in the menu (docked ships) and pressing X.
B) In space
Fly close to your ship, press the button to eject your current ship and then travel carefully to your other ship. Press "e" (default) to enter the ship. Make sure your current ship comes to a FULL halt before leaving it! Otherwise it might bounce you away or even kill you, depending on it's size ;).