You must first become a lawful permanent resident Green Card holder. In most cases, this is the first step before applying for U.S. citizenship.

You need to have held a Green Card for at least 5 years.If you're married to a U.S. citizen, the requirement is only 3 years.

You must live in the U.S. for most of that time. You should not spend long periods outside the U.S. that break your continuous residence.