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.
The Art of Brom
Here we're getting into the subjective - Gerald Brom is one of my favourite artists (and a pretty fantastic novelist!). That said, if I recommended art books just for the beautiful images contained therein, my list of recommendations would be miles long.
The reason this book is close to my heart is because of its introduction, where Brom goes explains in detail just how he went from being an army brat to one of the most highly respected dark fantasy artists in the world today. I believe that one's work is flavoured by their life's experiences, and discovering the roots from which other artists hail can help give one perspective on their own beginnings, and perhaps their eventual destination as well.