Most people who wish to help their fiance(e) immigrate to US have a doubt on what form to file, whether K1 visa or Form I 130. It depends on the the length of the relationship, the supporting evidence, the number of times the sponsor or the petitioner has physically met their fiance(e), are few of the considerations to file the appropriate form.
Only the citizens of the US may file a K1 visa for a fiance(e). If you are a legal permanent resident you may then have to file Form I 130 to petition your spouse.