There may be at least two processes that can help you sponsor your loved one for a green card in the US. There are advantages and disadvantages to each process.
If you are not married yet, you may be eligible to apply for a fiancé visa for your loved one. The advantage of the fiancé visa process is that your future spouse can be with you in the US while they wait for their green card application to be processed. Sometimes the wait for the green card application can take a long time and many couples really value the chance to be together during that waiting period. The fiancé process has more steps than the marriage based process and it can be more expensive. It can have major advantages if your fiancé has children who are approaching age 21.
On the other hand, you and your fiancé can marry now and apply for an immigrant visa. The advantages of this process is that it is just two-steps and less expensive in filing fees and attorney's fees. The downside for many couples is that the processing time is generally spent with you and your spouse living apart in different countries.
Depending on your specific situation, there may be other advantages or disadvantages to one of these processes. If your fiancé will need a waiver for inadmissibility, one process may be better than the other. If you are not sure which process makes the most sense for you, talk to an experienced immigration attorney.