With melodies and harmonies, unless you have decent understanding of theory and/or are a instrumentalist of some sort, it'll come easier to you by just messing around. A lot of people come up with melodies and chords by playing in random notes, moving them around, and eventually they find something they like.

One thing you could note is what notes are in the key of your song, then messing with those notes specifically. That way, you know the notes you're playing will all sound good

if you listen to a lot of future house melodies it uses a lot of quick notes and then they usually have some pauses throughout the progression. Tchami's Untrue is one of the best examples I can think of here. you can hear that he has notes really quick after each other and then those pauses that create tension. that plus knowing theory and creating tension with the actually notes your using will definitely help.

https://www.youtube.com/watch?v=DPHsToh4z3Y