Question

where should we use had or have

Answer 1

Had is used in past perfect to indicate the past action but have is used in present perfect to indicate present action

Answer 2

Had is used with past tense whether it is singular or plural noun.
Example:She had done the work.
In this SHE is singular.

Have is used in present tense with plural noun.
Example:They have done the work.
Here they is plural noun.

You can't say it like she have done the work. This is a wrong sentence.

And yes 'I' is exception. We use it like I HAVE DONE THE WORK.

Hope it helps