Here are some of the legal requirements for immigration into the U.S.
http://uscis.gov/graphics/howdoi/LPReligibility.htm
Who May Apply to Become a Lawful Permanent Resident While in the United States?
You may be eligible to apply for adjustment to permanent resident status if you are already in the United States and if one or more of the following categories apply to you.
Family Member
You are the spouse, parent, unmarried child under age 21, the unmarried son or daughter over age 21, the married son or daughter, or the brother or sister of a United States citizen and have a visa petition approved in your behalf.
You are the spouse or unmarried son or daughter of any age of a lawful permanent resident and you have a family-based visa petition approved in your behalf.
Employment
You are an alien who has an approved visa petition filed in your behalf by a United States employer. For more information on how an employee can become an immigrant, please see How Do I Apply for Immigrant Status Based on Employment? In addition, please see our list of ?How Do I? pages which provide information on bringing relatives (Parents, Spouses, Siblings, Children) to live in the United States.
Visa Number
If you are a Family- or Employment-based applicant, you must have an immigrant visa number available from the State Department unless you are in a category that is exempt from numerical limitations. Immediate relatives of United States citizens are exempt from this requirement. Immediate relatives of U.S. citizens are parents, spouses, and unmarried children under 21. (For instance, you can apply to adjust to permanent resident status at the same time that your U.S. citizen daughter files an application for you to become an immigrant.)
Other immigrant categories that are exempt from numerical limitations and do not need a visa number include special immigrant juvenile and special immigrant military petitions. For more information on immigrant visa numbers, see How Do I Get an Immigrant Visa Number? Also see How Do I Bring My Spouse to the United States to Live? How Do I Bring My Child to the United States to Live? and How Do I Bring My Parents to the United States to Live? USCIS Form I-360 provides more information on special immigrant juvenile and special immigrant military petitions.
For the unmarried son or daughter (over 21 years of age) of a US Citizen, brother or sister of a US Citizen, or the spouse or children of lawful permanent residents, visa numbers are limited by law every year. This means that even if the USCIS approves an immigrant visa petition for you, you may not get an immigrant visa number immediately. In some cases, several years could pass between the time the USCIS approves your immigrant visa petition and the State Department gives you an immigrant visa number. For more information on visa numbers, please see How Do I Get an Immigrant Visa Number?
Fiance(e)
You were a fiance(e) who was admitted to the United States on a K-1 visa and then married the U.S. citizen who applied for the K-1 visa for you. (If you married the U.S. citizen but not within the 90-day time limit, your spouse also must now file USCIS Form I-130, Petition for Alien Relative ). Your unmarried, minor children are also eligible for adjustment of status. See How Do I Bring My Fiance(e) to the United States? for more information. If you did not marry the U.S. citizen who filed the K-1 petition in your behalf, or if you married another U.S. citizen or lawful permanent resident, you are not eligible to adjust status in the United States.
Asylee
You are an asylee or refugee who has been in the United States for at least a year after being given asylum or refugee status and still qualify for asylum or refugee status. See Asylee or Refugee Seeking Lawful Permanent Resident (LPR) Status, How Do I Apply for Asylum?, and How Do I Get Resettled in the United States as a Refugee? for more information.
Diversity Visa
You received notice from the Department of State that you have won a visa in the Diversity Visa Lottery
Cuban Citizen
You are a Cuban citizen or native who has been in the U.S. for at least a year after being inspected, admitted, or paroled into the United States. Your spouse and children who are residing with you in the United States may also be eligible for adjustment of status.
U.S. Resident Since Before 01/01/72
You have been a continuous resident of the United States since before January 1, 1972. See 8 CFR 249.2(a), under 'Jurisdiction.'
Other Nationality-Based Programs
Parent's LPR Status
Your parent became a lawful permanent resident after you were born. You may be eligible to receive following-to-join benefits if you are the unmarried child under age 21 of the lawful permanent resident. In these cases, you may apply to adjust to permanent resident status at the same time that your parent applies for following-to-join benefits for you. For more information, see How Do I Bring My Children to the United States to Live?
Spouse's LPR Status
Your spouse became a lawful permanent resident after you were married. You may be eligible to receive following-to-join benefits. In these cases, you may apply to adjust to permanent resident status at the same time that your spouse applies for following-to-join benefits for you. For more information, see How Do I Bring My Spouse to the United States to Live?
Otherwise Eligible Immediate Relatives
If "otherwise eligible" to immigrate to the U.S., immediate relatives may adjust status to LPR (get a "green card") in the United States even if they may have done any of the following:
worked without permission,
remained in the U.S. past the period of lawful admission (e.g., past the expiration date on your I-94) and filed for adjustment of status while in an unlawful status because of that,
failed otherwise to maintain lawful status and with the proper immigration documentation, or
have been admitted as a visitor without a visa under sections 212(l) or 217 of the Act (which are the 15-day admission under the Guam visa waiver program and the 90-day admission under the Visa Waiver Program, respectively).
Please note: If a person came into the U.S. illegally, that person is barred from adjusting their status to LPR (cannot obtain a green card) even if he or she marries a U.S. citizen or otherwise becomes an immediate relative. An immediate relative must meet the eligibility requirement of being 'inspected and admitted or paroled into the United States.'
Ineligible
There may be other reasons that you are eligible for adjustment to permanent resident status. Please see USCIS Form I-485 for more complete information.
You may be ineligible for adjustment to permanent resident status if:
You entered the U.S. while you were in transit to another country without obtaining a visa.
You entered the U.S. while you were a nonimmigrant crewman.
You were not admitted or paroled into the United States after being inspected by a U.S. Immigration inspector.
You are employed in the United States without USCIS authorization or you are no longer legally in the country (except through no fault of your own or for some technical reason). This rule does not apply to you if:
You are the immediate relative of a U.S. citizen (parent, spouse, or unmarried child under 21 years old).
Certain foreign medical graduates, international organization employees and family members.
You are a J-1 or J-2 exchange visitor who must comply with the two-year foreign residence requirement, and you have not met or been granted a waiver for this requirement.
You have an A (diplomatic status), E (treaty trader or investor), or G (representative to international organization) nonimmigrant status, or have an occupation that would allow you have this status. This rule will not apply to you if you complete USCIS Form I-508 (I-508F for French nationals) to waive diplomatic rights, privileges and immunities. If you are an A or G nonimmigrant, you must also submit USCIS Form I-566.
You were admitted to Guam as a visitor under the Guam Visa Waiver Program. (This does not apply to immediate relatives.)
You were admitted into the United States as a visitor under the Visa Waiver Program. (This rule does not apply to you if you are the immediate relative of a U.S. citizen (parent, spouse, or unmarried child under 21).)
You are already a conditional permanent resident.
You were admitted as a K-1 fiance(e) but did not marry the U.S. citizen who filed the petition for you. Or, you were admitted as the K-2 child of a fiance(e) and your parent did not marry the U.S. citizen who filed the petition for you.
There may be other reasons that you are ineligible for adjustment to permanent resident status. Please see USCIS Form I-485 for more complete information.
