I think you can say 당신은 춤을 잘 추네요/ 당신은 잘 춤추네요 or just 춤 잘 추네(요) ^^

In your case, you seem to say that to k-popstar. If it's right, '오빠는 춤을 잘 춰요.' is much better. Korean guys loves to hear the nickname '오빠'. It doesn't matter they are biological brothers or not. Even grandpa will be happy to be heard it. lol I saw lots of korean learners using '당신' in this site. Don't try to use the word'. We don't use '당신' in conversation often. You can see the word '당신' in music lyrics or novel books, formal docs, when people fight. ex) 당신이 뭔데! or the name of senior people. mom was upset at mom-in-law. mom is explaining to dad what grandma did to mom. dad: 어머님이 왜 화나셨어? (how come mom is upset?) mom : 당신이 먼저 화를 내셨어. (She was upset at me first). we don't say 그녀가 먼저 화를 내셨어. I hope this is clarified using '당신'.

I Was thinking that 당신 is a formal way to say 너 .. what's 당신' s exact meaning ??

The sentence needs a verb. Also, who are you speaking too? This will determine formality. 당신 is not really used as you like it's taught. Use 너 (you informal) or the person's name or title.