developing a good hearing for notes and improvised things comes with time. in the beginning i also had problems to figure otu a melody no matter if it was a melody from an existing song or if it was something that i just had im my head. but with time and practice you´ll develop a feel for it. i suggest you put on a song that you like, and try to play along with it - not the actual melody - just improvise something - you´ll realize what fits and what not - this makes you comfortable with impriovisation and trains your ear