。
I'm thinking of going to the dentist tomorrow.
Literal
Tomorrow dentist [to-に] examine-receive-[volitional-もらおう] [quotative-と] thinking.
診てもらおう uses the volitional of ~てもらう — 'I'll have (the dentist) examine me.' ~てもらう marks receiving a service or favor, and the volitional ~おう wraps it in 'I think I'll...' via ~と思っています. 歯医者さん adds the respectful さん to 'dentist.'