An immigrant (also called a "lawful permanent resident") is a foreign national who has been granted the privilege of permanently living and working the the United States. If you want to become a lawful permanent resident based on the fact that you have a relative who is a citizen of the United States or a relative who is a lawful permanent resident, you must go through a multi-step process:
You must be eligible for lawful permanent residence based on a family relationship.
The U.S. Citizenship and Immigration Services (USCIS) must approve an immigrant visa petition, Form I-130 Petition for Alien Relative, for you. This petition is filed by your relative (sponsor) and must be accompanied by proof of your relationship to the requesting relative.
The Department of State must determine if an immigrant visa number is immediately available to you, even if you are already in the United States. When an immigrant visa number becomes immediately available to you, it mean that you an apply to have one of the immigrant visa numbers assigned to you.
If you are already in the United States, you may apply to change your status to that of a lawful permanent resident after a visa number becomes available for you. If you are outside the United States when an immigrant visa number becomes available for you, you must then go to the U.S. consulate servicing the area in which you reside to complete your processing.