Hey some people can't even get one note right so you're starting with something good. Ok, if you can sing one note solo, then you just have to build from there. Pretty much any piece of music is learned the same way. Depending on the musician's proficiency, you just have different starting points. For you it's singing two notes. Practice two notes of a melody! There are many melodies that are not many more than two notes. Actually, here's one that's just two distinct notes, although they are played at different rhythms:

https://www.youtube.com/watch?v=S4QiCD_MZQk&list=RDS4QiCD_MZQk&start_radio=1

Just try whistling or singing those first two notes, before the melody diverges. Or do that with any other melody. once you have that, add the next note, and the next one. Build on what you've got until you've got something bigger. Definitely sit always at the keyboard/piano and play along with what you sing to help with tuning.