When 'have' is the main verb, you conjugate it in the simple tenses in the same way as any other verb, with do/does/don't/doesn't/did/didn't. For example.:
I have a car --> I don't have a car
He has problems ----> He doesn't have problems
I have a coffee break at 11 am ----> I don't have a coffee break at 11 am
She had her baby in hospital ----> She didn't have her baby in hospital
When 'have' is an auxiliary verb, you conjugate it like other auxiliary verbs, by adding 'not' or the contraction 'n't'. For example:
I have seen that film ---> I haven't seen that film.
He has been to work ---> He hasn't been to work,
They had met before ----> They hadn't met before.
Note that in sentences in the present simple with 'have' + 'got', the verb 'have' is an auxiliary verb. This means that you say 'haven't' rather than 'don't have'. For example:
I've got a headache ---> I haven't got a headache.
He's got the answer ---> He hasn't got the answer.
These rules also apply to question forms. You ask 'Do you have..?' when 'have' is the main verb and 'Have you ...?' when it's an auxiliary verb.
I hope that helps.