Oh Christmas Tree and I Saw Three Ships are the first two songs for YuleUkers to learn. Each has just 2 chords (F & C7) and the music is on pages 12 and 13 in the 21 Easy Ukulele Songs for Christmas book. Once you’ve purchased the book you may also link to the video lessons which are here.
Here is an audio file of Oh Christmas Tree to practice with.
Here is an audio file of I Saw Three Ships to practice with.