How do I build a realistic-looking spaceship

The late Douglas Adams described the Heart of Gold in The Hitchhiker’s Guide to the Galaxy as looking like a training shoe. The Sulaco from Aliens looks like the cross-product of a sexual encounter between a slug and a shotgun. There is no real right or wrong; so for something realistic, you need to look at what gives spaceships realism. Well, the most important thing is aerodynamics. It might fly up there in the vacuum of space, but down here, where you have climate, friction and gravity, things can get a little difficult if your ship is shaped like a half-peeled potato and weighs in at around one million tonnes.