Hm.... Tough.
I'd make sure that I had my saxophone with me when I went to termina. Then I'd slap the Happy Mask Salesman and make him teach me the song of healing. After learning how I'd go to Hyrule and make Zelda teach me the Song of Time. I'd then return to termina, play the Song of Time in the nick of time and begin on the same quest. Of course, I'd also make people sew me green clothes just to make it authentic.
If none of that worked, I'd retreat to Hyrule and hope that there was a time zone change so that I'd have enough time to free all the Sages and get them to seal the moon and Majora's Mask in the Sacred Realm with Ganondorf. Sure, I'd need to defeat him too, but I'd call everyone here on ZD and get them to help out.