Once you are the beneficiary of an approved immigrant petition and an immigrant visa number is immediately available to you, there are two ways to apply for lawful permanent resident status (a Green Card). If you are outside of the United States, you may apply at a U.S. Department of State consulate abroad for an immigrant visa in order to come to the United States and be admitted as a permanent resident. This pathway is referred to as consular processing.
If you are already in the United States, you can apply for permanent resident status without having to return to your home country to complete processing. This process is called adjustment of status.
If you are in the United States, but you entered without inspection (crossing the board), you will not be eligible to adjust status in the U.S. Your case will be sent to the embassy in your home country. A waiver will be necessary.