Can I get him a fiancé visa even if I’m only 18?

Yes. Generally, as long as the couple is of legal age to marry (and not ineligible to marry for other reasons), the U.S. citizen may petition for her fiance to get a K-1 fiance visa for the purpose of entering the U.S., becoming married within the required time, and completing the Adjustment of Status application process so the foreign national spouse may become a Lawful Permanent Resident (get a "Green Card"). Some immigration law firms, including mine, offer legal services on a "flat fee" basis so that a client will know the total expense from the very beginning, and a few immigration law firms, including mine, offer an initial consultation free of charge.