My unasked for advice on coming up with melodies if you've never done this before: just play the instrumental in the background and make anything up over it. Don't worry about words or anything just sing "la la la" or whatever. Then if you do something and you're like "wait that was cool" remember it and that can be your melody!
The best melodies are fun to sing. Think of how much more fun it is, for instance, to sing Teenage Dream than This Is How We Do.
