"Do you want to go to a theme park with me tommorrow?"
'Sorry but I have something to do tommorrow."
Does this answer(the way I word it) sound strange to native speakers' ears?
First, note the correct spelling of "tomorrow".
Second, your answer is OK. You will probably hear "Sorry, I'm [already] doing something else tomorrow" or "Thanks, but I'm busy tomorrow" more often.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.