Cracked plaster is a sign that your house might be settling. Luckily, it's pretty easy to fix with just a couple of things:

Use a utility knife to clean away loose plaster. After that is done, vacuum all of the dust out of the crack.

Wet a small cloth or paintbrush with some water and run it along the length of the crack.

Use your putty knife to push enough spackle into the crack to fill it up.

Smooth the repair out by running the putty knife at a slight angle along the length of the crack.

Let the repair dry for a couple of hours. When dry, check the crack to make sure that the putty didn't shrink while it was drying. If it did, just repeat the process.