First of all,
When you are asking what they are doing, what do you say? If you say 뭐해(Muohae)?, you are not cute. You should say 모해(Mohae)? or 모행(Mohaeng)? with a smiley face ^_^.
When you are saying you were thinking about him, what do you say? 오빠 생각하고 있었어(Oppa Saeng-gak-hago iseoseo). No, no, no, no!!!! 오빠 생각하고 있쪄쪄(Oppa Saeng-gak-hago ijjeojjeo) like a baby trying to talk. You can change everything from '~었어(seoseo)' to '~쪄쪄(jjeojjeo).'
Try these! Your oppa will love you more!!!! hahahaha.