Having trouble coming up with something to draw? No worries - while you'll eventually learn how to start from a tiny seed of a thought and gradually nurture it into a complex concept to explore through design and illustration, it's perfectly fine not to be there just yet.
For now though, here's an idea that might interest you.
You're sitting in the tub. The water's perfect, but you're soooo bored. So, you start spying the things around you, and like a good little designer, your gears start turning.
Take something you'll find in a bathroom, and turn it into a spacecraft of some sort. Pay attention to the shapes and forms present there, and the proportions of them. Think about how you can push and pull, stretch and squash, and repeat them as needed to bring life to your design.
Sometimes the best designs come from the kinds of objects that don't look like your desired end result at all - so don't feel like you have to find something that already looks like a space ship. The fun and creativity comes from how you get from here to there.