There are SO many ways to say "you" in Korean.
This video will discuss the most common ways, including 당신, 너, 그쪽, using familiar terms (오빠, 형, 언니, 누나), using titles, and using names (among other ways).
If you don't have time to watch this video, here's a quick summary:
Don't use 당신 to mean "you." 너 is very rude except with your closest friends who are also the same age as you or younger. Use a person's name if you're friends. Use a person's title if you're not friends. Use 그쪽 to complete strangers.