it's him.
it's he.

it's her.
it's she.

how do i figure out what to use? someone pls help. thanks in advance.
You have added problems here.

it's us.
it's we.
The correct versions are "It's he", "It's she", "It's we" and "It's I." Whenever the verb is a form of "to be" (be, am, is, are, was, were, been), the predicate is subjective.

That's standard English. Most speakers of the language, however, would use the incorrect version in conversation. "It's me" just sounds better than "It's I."
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Grammatically, the subjective predicate should be in the subject case such as:
It's I/we/he/she/they. However, this form is rarely used in the colloquial English.
Most people would say: It's me/us/him/her/them.
unless you have to use formal English.