where is the bathroom? : 화장실 어디있나요? i need to use the bathroom : 화장실 가고싶어요(화장실에 가야해요) so if you need to use the bathroom, you can say "where is the bathroom(or toilet)?"

Where is the bathroom is: 화장실 어디있어? Or (저는) 화장실 어디있어요? I need to use the bathroom is: 저는 화장실에 갈 필요가 있어요./ 저는 화장실에 갈 필요해요. Or 나는 화장실 갈 필요해 Depending on how respectful you want to be

욕실이 어디예요? (Where is the bathroom?) 욕실 쓰고 싶어요/화장실 써야 해요. (I need to use the bathroom.) 욕실 means bathroom, but Korean houses nornally contain a loo in the bathroom, so you can say 화장실 instead. But when you mean only BATH not LOO, then you have to use 욕실.

Where is the bathroom = 화장실이 어딘가요? [hwa jang sil i eo din ga yo?] I need to use the bathroom = 화장실에 가야해요. [hwa jang sil e ga ya hae yo.] Bathroom = 화장실 [hwa jang sil]