As an adjective, you have two choices: "scared" or "scary."
Why does she have that scared look on her face?
They look very scared. What’s wrong?
Nightmare on Elm Street is a scary movie.
That geometry test was scary. ("Scary" sometimes means "difficult," "unusual," or "weird.")
Sheila’s hair looks kind of scary today.
Who’s that scary guy that lives in the old house down the street?
Sometimes "scare" is used as a noun:
I had a scare last week when I went to the doctor and found out that my blood pressure was way to high. (a scare = a frightening result, often related to one’s personal health and well being.)
This will put a scare into you. (This will frighten you.)
A note to other teachers: It always surprises me to learn how many of my students don’t know this word. They know "afraid" or "fear," but they don’t know "scare" or "scary."