@NKM-oct23: Yes my father is a US citizen. On Feb 2001 i was under 18.
I have to wonder why nkm asked that, as that fact should be obvious based on you being currently 21.
@Huracan: Yes my father is a US citizen and he did get naturalized before my 18th.
@Jackolantern: Yes i did have a green card when i was under 18 whilst living with a US citizen parent.
Assuming you mean living
within the US with your US citizen parent (presumably your father), and he had legal custody of you in addition to you physically living with him, all indications are that you became a citizen when he naturalized (or when you obtained your green card and started living with him in the US, if that began after he naturalized).
For proof of legal custody, you need the court papers showing the custody order if your parents are divorced or separated, or proof that your parents are still married and living together (if married and living together, custody is assumed for both parents).
For proof that you were living with him, you need documents showing that you had the same address as him when you were under 18, such as school records.
Go ahead and apply for the passport! Once you get the passport, also consider applying for the optional N-600, which is a certificate of citizenship that does not expire, and it serves as the root document for proving your citizenship if your passport is lost or its validity is challenged. Without that certificate of your own, the root goes back to your father's naturalization certificate and the custody papers.