I think melodies is one of the most frustrating things a musician can endeavor into, and I'm no different. But I've found that using the keyboard is the most beneficial, especially if you're not an experienced singer (which I'm not). I'd say play chords in your left or right hand to a beat or a rhythm that feels good and then mess around with notes in the same key in the other hand. Brian Wilson reportedly uses "feels" to write songs, meaning that he would just sit at the keyboard and feel around in the notes for a while until something caught his attention, then he would work with that. It may not be Paul McCartney coming up with "Hey Jude" in his head on the way to see Julian, but it's a method.