If you are an immediate relative of a U.S. citizen, you can become a lawful permanent resident (get a Green Card) based on your family relationship if you meet certain eligibility requirements.
You are an immediate relative if you are:
The spouse of a U.S. citizen;
The unmarried child under 21 years of age of a U.S. citizen; or
The parent of a U.S. citizen (if the U.S. citizen is 21 years of age or older).
If you are currently in the United States and you meet certain other requirements, you may file adjustment of status application (I-485) to apply for a Green Card without leaving the country.
As an immediate relative, you may file your I-485 application together with the Form I-130, Petition for Alien Relative filed on your behalf, while the Form I-130 is pending, or after the Form I-130 is approved (and remains valid).
Immigrant visas for immediate relatives of U.S. citizens are unlimited, so the visas are always available.