So, if you were expecting a tutorial on how to draw a horse or something, sorry. That's not here yet. This is a couple of methods that can help you to get ideas into your head, and all that stuff.

Get a piece of paper and just draw. Go right ahead, without thinking about the ideas. Obviously put in some effort, you don't want a whole bunch of scribbles.
If you have an idea, draw it. No matter how ridiculous or stupid it seems. Trust me, it works- by doing this you can unleash your creative flow and better ideas will come to you later. Don't block your ideas or change them when doing this. Remember, this is just a small activity. It won't be shown to the public on big billboards or plastered on popular websites. You can draw a rhinoceros on a duck in space that's fighting a lobster with a bottle of barbecue sauce and nobody will see it. Don't worry!

Another tip for drawing what's in your head is to draw in your peripheral vision. Point your eyes away from the paper or sketch, you can allow your vision to become blurry if you want. As the pen moves, try to trace along the lines in your mind. The result may not be exactly how you imagined it, but no work of art ever is, right? :)