Which one is correct and why? Thanks :)

Hi Joey! Jen already told you that 1. is correct (also, use periods after your numbers!). As to 2., the reason why is because, unlike 1., it does not come from an affirmative sentence. Generally speaking, to ask a question in English we typically use one of the auxiliary verbs (be, do, have) or a modal verb (can, will, may, etc.). And, if you expecting a yes or no answer, then the question starts with the auxiliary or modal. Without this subject-auxiliary inversion ability, something is grammatically wrong. So:

Affirmative sentence: She is good at cooking.--YES   Question: Is she good at cooking?--YES (your above)

Affirmative sentence: She does good at cooking.--NO  Question: Does she good at cooking?--NO (yours above)

Affirmative sentence: She have good at cooking.--NO   Question: Have she good at cooking?--NO

Now, you can easily see that you were trying to use the same phrase (good at cooking), yet, using a different auxiliary--"do" (or "have" in my last example). While English is somewhat flexible, certain phrases only work with certain auxiliaries, which is precisely the reason different ones, along with modals, exist. In short, you were mixing apples and oranges.

If you use one of the other auxiliaries, you could still ask a similar question, but the meaning would be slightly different. Thus:

Affirmative statement: She cooks well.   Question: Does she cook well? (Here, we used "does" a form of "DO" because they is no auxiliary in our original statement. This is known as do-support. Otherwise, we would end up with "Cooks she well?" which is not a grammatical sentence.)

Affirmative statement: She has cooked before.   Question: Has she cooked before? (Here, we used "has" a form of "HAVE")

Again, both of the rules above are evident here. I trust that is is clear to you now, but my sincerest apologies if I only succeeded in confusing you more. (*_^) I hope that this helps you. J.