One of the easiest ways to build a melody is using "Triad-Melody Technique". Triads offer multiple combinations of organizing the three chord-notes through repeats, inversions and rhythmic patterns. One of the most well-known examples are the big band era #1 hit "In the Mood" (composed by Manone, Razaf and Garland; released 1939 and made famous by the Glen Miller orchestra). Notice and listen, starting measure 9, how the triad is simply arpeggiated and rhythmically displaced across the 4 beat measure.
In "Music Composition for Teens - first course" chapter 9 is devoted to creating melodies using triads. Check out that publication (on amazon or this site) with multiple lessons, photocopiable pages, listening ideas, performance ideas and fun lessons including an Etude, Piano melody,Boogie, Piano Piece, Strathspey, Fanfare, Handelian Recitative, Jazz-A-Tazz, Try Triads, and an Easy Listening Song!