If you are a US Citizen, you may petition your spouse, parent and children for a legal permanent resident (green card) status. As as green card holder, you can only sponsor your spouse and unmarried children.
The first step is the “Immigrant Petition” that will establish a qualifying relationship between the you and your child abroad. The next step will be the green card application. If you are a US Citizen and your child is in the US, then you can file the Immigrant Petition and Green Card for children application simultaneously. But as a lawful permanent resident, your child must wait for the Immigrant Petition to be approved before you can go ahead with the green card application.
Firstly, you have to file Form I-130, the Immigrant Petition. If your child lives outside the US, then the next step in the process is to make the application for the Immigrant Visa at a US consulate which has jurisdiction over your child’s place of residence.
If your child lives in the US, then you file Form I-485 through which you can adjust your status. As a US Citizen, and if your child lives in the US and is both unmarried and under the age of 21, then you can submit Form I-130 and Form I-485 at the same time. This is supposed to be a faster way to apply for a green card. But if you are a Legal Permanent Resident and your child is married or over 21 years old or lives outside the US, then you must wait for the Form I-130 to be approved before moving to the next step.
If you are a US Citizen, then you may also sponsor your married child for a green card. Married sons and daughters of US Citizens come under the third-preference category. In other words, they have to wait for a priority date to become available before they can apply for a visa meaning married sons and daughters must wait longer that unmarried sons and daughters to apply for a green card.
Legal Permanent Residents do not qualify to sponsor their married sons and daughters for green card status.
US citizen parents can sponsor their unmarried children below the age of 21 as “Immediate relatives”. They can directly start applying without having to wait for a priority date to become current. The US citizen parent can also sponsor their children who are married or over the age of 21, however these children will not qualify as immediate relatives. These applicants have to wait for their priority date to be current before they are eligible to apply.
A monthly visa bulletin published by the Department of State has details for applicants when their Priority Date is current. Before applying for a green card, applicants must wait for their Priority Date to become current.