If you have a fiancee that is not a US citizen and you wish to take her back to the United States for the purposes of marriage then you need to apply for a US Fiancee Visa also known as a K1 Visa.
There is a time limitation once you have the K1 Visa. This visa is only there to ensure that you can arrive in the US with your fiancee and marry her. Once she enters the United States you will have to get married within 90 days of arrival. Once you are married she can apply for an Adjustment of Status and then to a permanent resident. Any children that there might be will need to have a K2 visa. These children or child must be a minor and single. You cannot arrive in the US with a 17 year old who got married. This would not be applicable under the K2 visa rules.
This is a once off visa. This means that once she is in the US she will not be able to leave the US until she has married else the visa will be cancelled when she leaves the country. She can apply for Advance Parole before she leaves the US and will be able to return to the US to continue the marriage process. Note, that a resident of the US cannot apply for a fiancée visa for his/her spouse. You need to be a US citizen to make this application. Also note that you can only do this 3 times without getting married before the US government bars you from applying again.
What Is a K-1 Visa?,