What you will need:
Autodesk SketchBook (This can be downloaded from autodesk.com. You can download an evaluation version of SketchBook Pro to try before you buy, or you can get SketchBook Express for free. This can be found on the Apple App store, Android store, Windows App store, possibly elsewhere. This is a scaled down version, but completely functional.)
A computer (SketchBook will also work on tablet computers, like the iPad etc.). I have a MacBook Pro. If you are using a PC, some of my shortcuts will be different since you don't have a command or option key.
Wacom drawing tablet. (Mine is several years old; I got it in 2001. It's an Intuos Graphics tablet. It's USB and it still works with my MacBook Pro, so if you can't afford a new one, you could find one used on eBay. If you are going to draw on your computer, it's essential.)
Step 1: Come up with an idea
Now, what to draw. First, you need an idea. My thought was to type in 'spooky' into Google and see what came up in the pictures. Haunted houses, some kids watching TV (?), moons, monsters, and lots of dark blue and black.
At least that gives me an idea for a back ground: a dark night sky.
So I selected a nice dark blue color from the color palette on the right, and used the paint bucket to paint the backdrop blue. Then I chose the marker, and changed the brush size by clicking in the 'Brush properties' circle, and dragging to the right to increase its size. I changed my color to black then I started coloring the edges, making it darker on the outside and lighter towards the bottom and middle.
If you look at night skies, they are typically lighter towards the horizon. If you mess up and make it too dark, switch to another tool, like the airbrush tool. The marker brush only makes things darker. I switched to the airbrush, increased it's circumference, and holding down the option key (which switches to the eye dropper tool) was able to select my blue background color, and go back over where I wanted it lighter.
Then switch back to the marker, choose black, and draw a horizon.