for a newbie

[Jeb]

Just to amplify a bit:

The easiest way to transfer cargo if you're in orbit around the planet in question is to use the transfer button, which looks like a green arrown and a yellow arrow pointing horizontally in different directions. If you're queuing orders, you can also use the yellow up arrow icon to order a ship to move to a planet and load population (or mines, satellites, etc), and the green down arrow to order a ship to move to a planet and drop cargo.