Scare is a verb that means frighten or intimidate. You might be embarrassed to admit that you don't want to go to the beach with your friends because of how much seagulls scare you.
Questions on a test you're unprepared for might scare you, and so might a barking dog. To scare is to spook, startle, or cause fear, and what scares your best friend most (like losing his job) might not be what would scare you more than anything (like discovering a huge snake in your shower). You can also use scare as a noun, as in "It gave me such a scare when all my friends jumped out and yelled, 'Surprise!'"