Here is the 4 most common ways most people obtain a green card
Green Card (Permanent Resident Card)
A Permanent Resident Card, commonly known as a green card, is evidence of your status as a permanent resident with a right to live and work permanently in the United States. It also is evidence of your registration in accordance with U.S. immigration laws. A green card is also called Form I-551.
Marriage – within a few months.
Fastest legal way as long as it is real. There will be financial obligations to the government for the citizen applying for the immigrant spouse green card and many other conditions. Even Gay and Lesbian immigrant married to US citizend now qualify for a Gren Card. Do not try to file papers yourself. Rules and regulations and caselaw changes all the time. Use an experienced immigration lawyer.
Extraordinary Ability – less than a year.
If you have extraordinary abilities in sciences, arts, education, business or athletics. If you have an award, doctorate, PHD, international acclaim, a Nobel, Pulitzer, Oscar, Emmy, Tony, Grammy or Olympic Medal this may be an option for you.
Asylum – within months.
Well prepared and documented Political Asylum applicants are approved about 50% of the time in the United States. You must show well founded fear of persecution based on race, religion, nationality, political opinion, or membership in a recognized social group, or fled a dangerous country because you were afraid of severe mistreatment. If this you have been in the USA for less than a year and this sounds applicable for you, then you should seek the advice of an experienced immigration lawyer as soon as possible to ensure your application is completed in a timely manner.
Diversity Visa Lottery – months to years.
About 50,000 people from certain nations, are able to acquire a green card through the Diversity Visa Lottery. Most likely success for this area will be for people with at least a high school diploma or skilled labor from almost any nation in the world. It is possible to apply for the Diversity Visa Lottery yourself but many intelligent immigrants seek the assistance of top immigration lawyers.
Summary
When applying for a Green Card, specific conditions will always apply to any of these methods. The best advice is to get help from a knowledable and experienced immigration lawyer.