Slang speech in English is considered non-standard. Colloquial expressions are standard though. They are considered correct English. But teachers usually don't like you using them. Actually, our teacher in the 8th grade pointed out, "ain't" sounds terrible. And people rarely use it today. But it actually is the correct form of "am not". Just like "won't" is the correct form of "will not". But all other forms of it, like in the sentence "ain't it the truth" are incorrect. But for "I am", it is considered standard English, even though it sounds incorrect. Now "it is me" is not correct. Me is in the nominative case because it is not receiving any action. And "it is me" was never correct, though everyone uses it. But "I ain't" would be considered correct. Historically at least. Although few people use it today.
