Some of the friends I have who write will listen to other songs they like and pick out the melody, then examine how it's put together -- it's rhythmic structure, what notes, how they relate to the chords, how they move, how they impact the lyrics. It can help to give you good ideas.
I studied counterpoint in college, there were some good rules there for constructing a melody that apply outside of classical music as well. I'll try to remember a few off the cuff...
1. Melodies usually have a high point, either at the beginning or the end. Don't use the highpoint at both ends, and don't revisit the high note in the phrase once you leave it.
2. Most melodies move in a stepwise fashion with some occasional leaps. Don't overuse leaps.
3. If you do a leap up, step down on the next note. If you do a leap down, step up on the next note.
There are of course notable exceptions to all 3 of those, the point isn't to slavishly follow the rules, but to use them as a starting point. You can make a "safe" melody using these rules.
In pop music, if you have a great chorus don't sweat the verse too much. Keep it simple and safe and let the chorus carry the song. Pop (and I'm using the term broadly here) does not have to be genius work from start to finish. Don't be afraid to be simple and do the obvious thing here and there.
