I don't know how to grow up

I'm 18 but I feel like my family thinks I'm still a child. I just graduated high school but I'm taking a gap year because I'm not ready for college. I'm going to work, but I feel like a burden and a deadbeat. My sister tells me to grow up because I get angry and yell at her a lot, but isn't that normal for anyone? Idk, I feel like a child and don't know how to act my age. Any advice??