If you wish to immigrate to the USA, you have to follow proper procedure and apply for a Permanent Residency visa before you can be legally considered as a permanent citizen of the country.
Family-based: Includes those who are immediate relative or a family member of a US citizen, a family member of a Green Card holder, or a member of a special category, including a person born to a foreign diplomat in the USA.
Employment-based: Includes those offered with a permanent employment in the USA, investors that help create new jobs, persons with extraordinary ability, or other special cases.
Refugee-based: Includes those who had been admitted in the USA as a refugee or a family member of an asylee a year ago and those who are granted asylum a year ago.
If you fulfill the following requirements, you are eligible to apply for permanent residence:
• Eligibility for one of the immigrant categories as established in the Immigration and Nationality Act (INA)
• Admissibility to the US
• Immigrant visa
• Qualifying Immigrant petition personally filed and approved
Once you've found that you're eligible and had all the requirements (documents and fees) in hand, just file an Application to Register Permanent Residence or Adjust Status (or Form I-485). United States Citizenship and Immigration Services (USCIS) review your application and notify about your interview schedule. Though the process seems simple, it is a bit tricky and people often need expert advice when applying. To avoid pitfalls, you should ask an immigration expert for guidance. Once the Green Card is issued, it'll be valid for ten years. When the ten years is up, you can just renew it.