In the key of C, the three notes that make up a C chord are the 1, 3 and 5 notes within the C scale. These notes are C, E and G. Every open note on your ukulele except for the A string or the first string, is one of these notes. G, C, E, but not A. To find a note that will harmonize with the song you are playing in the key of C, pick just the E string and finger your chords as normal. We tried it with "The Lion Sleeps Tonight." Pick the E string but finger the C chord. Sing the E string note you hear. Then move to the F chord finger while continuing to pick the E string only and sing the note you hear. Repeat for the C chord and finish with the G7 chord picking the E string only. This will give you a starting place to harmonize. You can then build your confidence to find intervals that are more interesting with the melody.