There's no one answer to this question since everyone's heart is unique and therefore will respond to different things. However, some general tips on how to melt your girlfriend's heart with words might includes expressing your love and appreciation for her, telling her how lucky you feel to be with her, confessing your deepest desires and feelings for her, and writing her a heartfelt love letter expressing your undying love for her. Whatever you do, make sure your words come from the heart and are spoken (or written) with sincerity.