I need to add chords to my melody to harmonize it????? HELP PLEASE?

Ok a have a melody in C major. I have chords written out and I want to harmonize my melody using C F G chords but I dont really know how to add these to the notes I have in my melody. For instance my first note is G as a 8th note and then an A as an 8th note then goes on to C G A to finish up my first bar....also sorry i cant be technically correct but the first four notes arent on the ledger lines they are like an octave below them or something and then the last A jumps up to the ledger lines..this is 4/4 in treble.... Do I like find the chords the fit with the notes in my melody and then just add them all together to harmonize? I want to know how to write this out on though so like if I have note of g as my first do i write an e and c below it? like do they have to stay in order or a b and d above it? like does the G have to stay as the note below the other two and the others build up above it??? i know im rambling but i just dont understand