Melody notes - check

[for]

So lets say i started writing a song and have only the melody line notes as per attachment


how do i decide which scale to adapt to from all the above for the whole song? as to make the best decision

[martifingers]

I think this is a question of Modes. The notes themselves don't define the key (if that makes sense!) Even assuming the song does not modulate (ie change key during the song) you can use the same notes but "start" from a different note thus resulting in different intervals in the notes of the scale. (this is called the root or "final" note).
Is that already too much jargon?

There's millions of tutorials out there but here's a fun intro to the subject: https://www.youtube.com/watch?v=Agln4_z8D8I&t=6m13s

Hope that helps (ands is accurate!)