When you determine the chords, look for the non-harmonic tones. There are notes that do not belong to the chords used in the harmony. The two main types of non-harmonic notes are passing tones and neighboring tones. Both are determined by the way they are approached and ended. You do not have to take these notes into consideration when you determine the chords. 

Passing tones: are notes that pass by between two chord tones.

Neighboring tones: are notes that are notes that leave and return to the same chord tone by step.

Take following sheet music "We wish you a merry Christmas" as an example. Notes A are passing tones; Note B are neighboring tones.

Following tips will help you to identify the non-harmonic tones:

1. Most non-harnomic tones occur on weak beat.
2. Look for the arpeggio’s pattern in the melody. The notes in the arpeggio can actually tell you the chords of the melody. Any other notes would be non-harmonic tones.
3. If a note is played more than once in different beat, it should be a chord tone. (jingle bell)
4. Most song end with the chord V resolving to the I chord.