"I weren't" is neither singular nor plural. It is WRONG (when used at the beginning of a sentence)!
"wasn't" is used with the first and third person singular pronouns (I, you, he, she, it).
"weren't is used with the second person singular pronoun (you singular, and the first, second and third person plural pronouns (we, you (plural), they).
Let me lay it out very clearly for you.
I wasn't
You weren't
He/She/It wasn't
We weren't
You weren't
They weren't
This is too complicated for you but "I were" and "I weren't" can be used in the subjunctive. For example, "If I weren't your friend, you would be very sad!"!
