Just change it into "I must go" - then it's easier to analyze:
"go" here is the full verb that is in its infinitive and follows the modal auxiliary must - without to!
I have to go:
"have to" is the substitute form for "must", "go" again is the full verb in its infinitive.
"Have to" here is a fixed expression, and therefore "to" is actually part of the following infinitive form of the full verb, not a preposition. If it was a preposition, usually a Gerund would have to follow (as eg. in "I look forward to seeing you).
To complete the analysis:
"I" is the subject of the sentence, it's a personal pronoun.
"have to go" is the predicate of the sentence which includes the inflected present tense form of "have" and the infinitive form of the full verb with "to".
"now" is an adverbial of time.
Hope that helped