Form N-600 is issued by U.S. Citizenship and Immigration Services (USCIS). It enables certain people who became U.S. citizens automatically, through their U.S. citizen parents or grandparents, to apply for a certificate proving their citizenship (although most applications involving grandparents are done on Form N-600K). The technical terms for gaining citizenship in such a way are either "acquisition" (when someone is born outside the U.S. to U.S. citizen parents) or "derivation" (when the parent(s) of a child who has a green card become U.S. citizens, most likely through naturalization).
The exact law that might apply to you cannot be summarized here, because it depends on the year in which you were born and the terms of the laws in effect at that time. If you're not sure whether you have in fact acquired or derived citizenship, speak to an immigration attorney.
Of course, you'll also need to prove your eligibility, by submitting supporting documents, as described in USCIS's instructions for this form, found via the USCIS page. Applying for a U.S. passport is a cheaper way to document your citizenship, but you will still need the same supporting documents that USCIS requires and there are some government agencies and employers that specifically request a Certificate of Citizenship.
Below we provide a quick summary of the most likely documents you'll need to submit and why they're important forms of proof.
You will, in the end, need to make your own judgments about which documents will prove that you have a valid claim to U.S. citizenship.
For example, if you were claiming to have acquired citizenship because you were born outside the U.S. to two married U.S. citizen parents between January 13, 1941 and December 23, 1952, you'd need to prove not only that both your parents were in fact U.S. citizens, but that one had a prior residence in the United States. In such a case it would be important to submit proof of both your parents' U.S. citizenship (such as their birth or naturalization certificates), proof of your relationship to them (your own birth certificate), and proof of one parent's residence in the U.S. (such as copies of a home mortgage, pay stubs from a U.S. employer, or the like).
If you were born outside the U.S. in 1990 to an unmarried U.S. citizen mother, the requirements would be very different. In addition to documenting your mother's U.S. citizenship and your relationship to her, you would also need to show that your mother lived continuously for one year in the United States before you were born.
The rules that determine whether children born abroad were U.S. citizens at birth can be vastly different depending on the circumstance of the birth. More information on this topic can be found in Automatic U.S. Citizenship for Children by Birth To Citizen Parents.
If you are claiming derivative citizenship because you are the child of a U.S. citizen that entered the U.S. on a green card while under 18, the documentary requirements will be different than those for someone claiming citizenship at birth.
Because your claim to citizenship rests upon your parent(s), you will almost certainly need to provide your birth certificate showing your parent(s) names along with your Form N-600. Adopted children can provide a copy of the final and full order of adoption.
All applications must also include two passport-style photos of the applicant with the person's name and A number (if the applicant has a green card) written lightly in pencil or ink on the back. You must also include proof of your parent(s) U.S. citizenship and the birth certificates for your U.S. citizen parent(s) for all types of applications.
For a claim based on citizenship acquired through birth abroad to U.S. citizen parents, it's possible that one of your parents might never have claimed U.S. citizenship, having gained it through their own parent(s). If you are relying on that parent's U.S. citizenship for your claim, you would also need to provide proof that your parent met the requirements to acquire U.S. citizenship through your grandparents and proof of your grandparents' U.S. citizenship.
Any documents that show different last names, for you or for your parents, will need to include proof of how the name change occurred (marriage, divorce, legal name change).
If you are claiming you acquired citizenship at birth and your parents were married, you'd want to submit their marriage certificate.
If your parents were not married, but you were legitimated by your U.S. citizen father at a later time, you'd need to provide proof that your father complied with the legitimation laws of your home country, such as perhaps signing an affidavit confirming his parental financial responsibilities. If your U.S. citizen mother was never married to your non-U.S. citizen father, you can file a claim through your mother if she meets other requirements, and you would not need to send a marriage certificate.
If your parents were previously married to other people before they married each other, you would also provide your parents' divorce certificates.
If you entered on a green card and filed a derivative claim of citizenship, you also can use your parent's marriage certificate (if they were still married) to show that you were in the legal custody of your parents, which is a requirement for citizenship in this circumstance. If your parents are not married, you will need to show that your U.S. citizen parent had legal custody of you by presenting divorce or custody documents, or other proof of legal custody.
If you are claiming citizenship through acquisition at birth, you will need to show that your parents spent time in the U.S. The amount of time that must be documented depends on the circumstances of your birth. Typically documents such as school records, employment records, rental agreements, travel records, and vaccination records are used as proof of physical presence or residence in the United States.
For derivative claims, you will have to show that you were physically living with your U.S. citizen parent in the U.S., before your 18th birthday. You will need documents in your name and in your parent's name listing the same address. Driver's licenses, school records, employment records, and medical records are some of the documents that can be used to satisfy this requirement.
If the law in effect when you were born requires you to have been unmarried when your parents naturalized, then you should submit any of your marriage certificates, along with a certificate showing any instance in which your marriage was terminated due to divorce or death.
In rare circumstances, USCIS might request a certificate of single status from the civil registry in your home country.
Some of the laws on derivation of citizenship require you to have obtained a green card by a certain age, or before your parents became U.S. citizens. In such a case, you'll want to include a copy of your green card or other proof of permanent resident status along with your Form N-600. Your should also include a copy of your passport stamp showing the date you entered the United States on your green card.
All supporting documents accompanying the N-600 form should be readable copies. Don't send originals—you won't get them back.
If the documents are in a language other than English, those need to be translated, word for word, by a competent translator, into English.
The translator should include a signed statement on the bottom of the translated version saying: "I, [name] certify that I am competent to translate from [the language of the document] to English and that the above is a true and correct translation to the best of my knowledge and belief."
Below that, the translator should not only add a signature, but his or her address, telephone number, and the date.
If you have any questions at all about whether you have acquired or derived U.S. citizenship, or what documents will help prove your case, consult an immigration lawyer. The process of getting a certificate of citizenship can be lengthy under the best of circumstances. If USCIS isn't convinced by your documentary proof, the decision will drag on even longer, as it sends you letters asking for more documents, then takes time to review those documents, and so on. Having a lawyer's help will ensure that you present the best possible forms of proof to begin with.