Welcome to the first (and easiest) Christmas Carol.
Before you start, you should be able to already play a basic D shape and Major 7 barre chord. If you can't do this already, you're about to learn how!
Make sure to tune your guitar to Drop D. You'll have to lower your E string down to a D.
As you learn this song, be sure to recognise repeated phrases. This will save you a lot of time.