How to make a song in scale?

Let's say I am making a song and the four chords are D Maj-E minor-C
Maj-B Maj... and bass line is played the root note of every chord...
now, how will one tell what scale it is in? Plus, lets say if I choose
C# Maj Scale, how do you put the chords together with that?

Does that make sense? please help?
or better yet how would you make chords from this scale:

C - Dâ™­ - E - F - G - Aâ™­ - Bâ™­ - C