Lei17 -
Things you could study before training college include the Airport Codes of Emirates destinations. Phonetic Alphabet is handy too. You don't really get tested on these until 4-5th week of training college, but there's a lot to digest, so it's best to get these out of the way. Studying English would help too if it's your 2nd language. Maths you don't need until business promotion.
It's rare that they reject you if you don't pass medicals unless its something major. If you need stuff done that doesn't get done in your home country they'll do it in Dubai and deduct it out of your salary.