In general, to become a United States citizen you must have been a permanent resident, which means have a green card for five years, and have lived in United States for most of that time, generally without any trips longer than six months in length. There are some exceptions where if you are married to a US citizen it could be after three years. If you serve in the military at a time of conflict it could be immediately. There are other circumstances such as military recruitment where you don’t even need to have residency or a green card in order to apply for naturalization, which is citizenship.