How do you say "It's going to be awesome!" and "You're going to be awesome!" in Korean?

재밌을 거야. [jae-mi-sseul geo-ya] 

잘할 거야. [ja-ral geo-ya]

How do you say “I'm sorry. I'm sick and I can't come.”in Korean?

= 죄송해요. 아파서 못 갈 것 같아요. [joe-song-hae-yo. a-pa-seo mot gal geot ga-ta-yo.]

How do you say “Bless you!” in Korean when someone sneezes?

= Korean people don't say anything when someone sneezes. ;-)

How do you say “I have two brothers; one 8 years old and the other 16 years old.” in Korean?

= [if you are the oldest] (저는) 남동생이 두 명 있어요. 한 명은 열여섯 살이고, 한 명은 여덟 살이에요. 

[(jeo-neun) nam-dong-saeng-i du myeong i-sseo-yo. han myeong-eun yeol-lyeo-seot sa-ri-go, han myeong-eun yeo-deol sa-ri-e-yo.]

[if you are the middle child] (저는) 오빠 한 명, 남동생 한 명 있어요. 오빠는 열여섯 살이고, 남동생은 여덟 살이에요.

[(jeo-neun) o-ppa han myeong, nam-dong-saeng han myeong i-sseo-yo. o-ppa-neun yeol-lyeo-seot sa-ri-go, nam-dong-saeng-eun yeo-deol sa-ri-e-yo.]

How do you say “I'm the only sister between two brothers in the family.” in Korean?

= 딸은 저 하나예요. [tta-reun jeo ha-na-ye-yo.] or 여자 형제는 없어요. [yeo-ja hyeong-je-neun eop-sseo-yo.]

How do you say “Please don't put onions in it.” in Korean?

= 양파는 빼 주세요. [yang-pa-neun ppae ju-se-yo.]