The most ovious reason is by birth. People who are born in the united states "jus soli; right of the land" or people who are born to US citizens "jure sanguinis; right through blood" are automatically US citizens.

      The second way to become a US citizen is by applying for citizenship through the Department of Justice of the Unite States. This is a long and grievous process. The migrants are know as aliens. Aliens are people who have left (emigrated) a foreign country to the United States. They have some of the same freedoms and legal rights as U.S. citizens, but they cannot vote in elections.

      The first step to becoming an American citizen is to receive a  LPR( Lawful Permanent Residency). They must then obtain a green card. This gives them some of the rights as a citizen. They must be a resident for five years. After five years they can apply for naturalization. There are six steps in applying for naturalization. First they must swear on an oath:

"I here by declare, on oath, that I absolutely and entirely renounce and abjure all allegiance and fidelity to any foregin prince, potentate, state or sovereignty, of whom or which I have heretofore been a subject or citizen; that I will support and defend the Constitution and laws of the United States of America against all enemies, foregin and domestic; that  I will bear true faith and allegiance to the same; that I will bear arms on behalf of hte United States when required by the law; that I will perform nomcombatant service in the armed forces of  hte United States when required by the law; that I will perform work of national importance under civilian direction when required by the law; and that I take this obligation freely without any mental reservation or purpose of evasion; so help me G*d"

    Then they must take a test of the US government and history. They must take a proficncy test to prove they can read, write, and speak English. They must fill out an application, be fingerprinted, and have their photo taken. Finally they must appear in front of a judge. The judge will then determine if they deserve citizenship. If so , they become citizens.