"Can hardly wait" versus "can't hardly wait"
Solution 1:
The phrase "I can't hardly wait" is incorrect.
I suspect it is the result of a confusion between:
I can't wait
and
I can hardly wait
which are both correct.
The phrase
I can't hardly wait
doesn't make sense: it would mean "I don't find it hard to wait", which is probably not what is meant.
Probably adding to the confusion is the 1998 teen movie "Can't Hardly Wait". It is possible that the title itself was picked up because the expression is in vogue in American high schools although I did not find any confirmation for this hypothesis.
Solution 2:
Both are correct in the sense that a native English speaker will understand what you meant when you use either.
The operative word in both expressions is hardly, which has multiple meanings:
- with difficulty
- barely, scarcely
The first sense of the word is what causes some to say can't hardly wait is incorrect: if you can't wait with difficulty, you must mean you can wait a good deal.
But in reality, it's the second sense of hardly that being used. Someone who can hardly wait has just enough willpower to wait out whatever it is. Someone who can't hardly wait doesn't even have that amount.
(Note: Wikitionary even claims the first sense is obsolete)