Which pitch to sing within a Chord

How does one know which pitch to sing within a chord?  For example, I'm looking at the lead sheet for Shake It Up, which shows the chord progression at the top.  When I sing it along with you, I have made a note of what pitches I am singing for each syllable, using a tuning app: 

C               Em                               Am

C-C-C-C-D-B-B-A-B-A-B-A-C

There's a fire deep inside, I can feel it turn the tide

             F

C-D-C, C-C-F-E

Oh-Oh-Oh, for the first time

On the part starting with "inside," on an E minor chord, you start with a flat 7th on the lower octave, then a 5th, 5th, then a 4th?, 5th, 4th, 5th, 4th.  These sorts of note choices happen again on the Am chord, and then the F right after that.  Is there any reason these pitches were chosen?  I mean, I know they sound good, lol, but, as a singer, if given a lead sheet like this, how would I know what to sing exactly?  Also, as a professional singer, are you always aware of what chord your bandmates are playing, and then only sing those pitches that are part of that? 

Maybe this touches on music theory a little, is there a reference for this somewhere, or a lesson?  Thanks!  This is exciting! 

Good question, Vik!

If you were a singer and you were asked to sing this song, you'd receive sheet music with the melody written out. You wouldn't be expected to come up with a melody yourself!

As for why certain pitches were chosen, "sounding good" pretty much sums it up! You'll notice that both the pitches of the melody and the pitches comprising the chords all fall within the key. In other words, it's totally diatonic. Often, a melody will come first, and chords to support or fit with the melody come second. The order is different depending on the song and the writer! 

Ah, ok, it is nice to know that a melody gets created first typically, then chords to fit around it next, in creative songwriting!  I tried writing a simple song recently, but just stuck to the root note of the chords for the whole melody, lol - so, this has expanded how I think of song lyrics, thank you! 

You certainly could start with chords and write the melody to fit within the chords. If you're starting with chords, I encourage you to build a melody using any pitches from the key you're in, and notice if it fits or clashes with the chords underneath. You'll find that almost any pitch (within the established key) will work, but some will sound better than others (at least to sustain or land on).