Melodies are meant to be memorable. Try a sentence or a verse you'd like to write music to.... it could be a verse from poetry. Now, when you're writing a melody for it, remember:
1) the way the melody rises and falls must FOLLOW the speech pattern, as in emphasis on the first or second syllable depending on the language. In English, most stresses fall on the first syllable.
2) whether the notes rise or fall depends on the mood you're trying to set. More climactic sections, like choruses, might have higher notes.
Once you try this exercise, shift your focus from writing melodies for lyrics, to writing melodies from scratch.