It would be more appropriate to say "I have a dentist appointment tomorrow" or simply "I have to go to the dentist tomorrow."

the proper way to say that is....(a) I have an appointment with the dentist tomorrow. (b) I have to visit the dentist tomorrow. Kindly revert to me in case you need any help in the english language section. I am from India and working as an american english trainer. thanks, Sharique