무서워요。 This is the simple answer. In Korea, there are different levels of speaking. To a close friend, you could drop 요 and just say 무서워. ex. I'm scared of dogs. 저는 개가 무서워요. Literally it means "As for me, dogs are scary". If you are scared of swimming, 저는 수영하는 게 무서워요. (As for me, the act of swimming is scary)

나는 무서워 / 나는 무서워요 informal / formal I 나 am -는 scared 무섭다

무서워요 (Museowuoyo).