If you have close family members in the United States, if they are willing, they may be able to assist you in obtaining a green card.  There are various factors to take into consideration, such as whether the family member is a United States citizen or green card holder, your and the family member's age, and the family member's relation to you (your spouse, brother/sister, parent, etc.). 

Immediate Relatives (Relatives Not Subject to Numerical Limitations)
Immediate relatives of United States citizens may immigrate to the United States in unlimited numbers and at the present moment they are not controlled by any annual limit or quota.  The following people qualify as immediate relatives:
     •  Spouses of United States citizens, including widows and widowers of U.S. citizens under certain
        circumstances
     •  Parents of United States citizens, if the U.S. citizen's child is 21 years of age or older. 
     •  Children of United States citizens, if the child is under the age of 21

Relatives Subject to Numerical Limitations
If you do not qualify as an immediate relative, as described above, you may obtain a green card as a result of being related to other types of United States citizens.  However, you will be subject to a preference order (waiting list) and numerical limitations, unlike immediate relatives who have no waiting list or numerical limitations.  The waiting list is based on various factors and is determined by the preferences below.  Immigration law sets forth the numerical limitation which limits the number of visas available. 

First Preference Relatives:  Unmarried Sons and Daughters of United States Citizens
Unmarried sons and daughters 21 years or older of United States citizens fall under the first preference category.  These sons and daughters of United States citizens qualify for the first preference of the relative immigrant visas (aka green card) that are subject to numerical limitations.

Second Preference Relatives:  Spouses, Children, and Unmarried Sons and Daughters of Green Card holders
Unlike any of the other preference categories, this category is the only one in which the family relationship is to a green card holder (aka long-term permanent resident).  Individuals who qualify under this category are the spouses, children, and unmarried sons and daughters of green card holders.  It is important to distinguish between "children" and "sons and daughters".  Children are unmarried and under 21, whereas sons and daughters are 21 or older, or married, or both.

Third Preference:  Married Sons and Daughters of United States Citizens
The married sons and daughters of United States citizens qualify under this preference.  Age is not a factor, as the son or daughter can be under or over 21 in this case.  The determining factor is that the son or daughter is married under this preference.

Fourth Preference:  Brothers and Sisters of United States Citizens
The brothers and sisters of United States citizens fall under the fourth preference category.  The United States petitioning citizen in this category must be over the age of 21 years.