What is Incompetech?
Incompetech [incompetech.com] is a website full of royalty-free music that you can use in your commercial projects, be they Youtube videos or other things, so long as you follow its Creative Commons license.
Incompetech's vast trove of music seems to have been created by a single person, Kevin MacLeod. It's worth noting that among MP3 files that you can download from the website for free, some of them are composed by other authors. For example, Jingle Bells is a very old song, so it's a work in public domain. However, anyone who records themselves playing Jingle Bells would have created a derivative work, which might met the threshold for a copyrightable work. This means even though Jingle Bells is in public domain, a particular MP3 of Jingle Bells isn't necessarily public domain. On Incompetech's website, you can find Kevin's version of Jingle Bells, which is licensed under creative commons, and you should be able to use it without trouble.
Incompetech's many audio files can also be added to your Youtube videos through Youtube's video editor. If you click to add audio to the video, you'll be able to search for "Incompetech" and find some of his compositions. I'm not sure if you're able to find all of them, though. Also, note that when you add music to your video in this way, you are legally required (by the license) to include attribution to the author, in this case, to Kevin MacLeod, but Youtube won't automatically add the attribution for you. It will warn you about it, and you have to manually copy the attribution and paste it in your video description.