An immigrant is a foreign national who is authorized to live and work permanently in the United States. You must go through a multi-step process to become an immigrant based on employment.
1. The USCIS must approve an immigrant petition (application) that was filed for you, usually by an employer.
2. In most employment categories (See EB-2 and EB-3 eligibility and filing information), a U.S. employer must complete a labor certification request (ETA 750) for you from the Department of Labor's Employment and Training Administration.
3. The State Department must give you an immigrant visa number, even if you are already in the United States. Please see How Do I Get an Immigrant Visa Number?.
4. If you are already in the United States, you must apply to adjust to permanent resident status when a visa number becomes available. Please see How Do I Become a Lawful Permanent Resident while in the United States? If you are outside the United States when an immigrant visa number becomes available, you will be notified to go to the local U.S. consulate to complete the processing for an immigrant visa.
Source:
http://www.uscis.gov