You need to hunt down royalty free music - so they don’t have a Copyright against them basically and that the creator of the music has given it a creative commons license for it to be used ( generally for a non commercial use )
YouTube has a load of free music that can be used though I have never found anything to my liking on it - you can find that in the YouTube Studio page or here. I have also found Soundcloud has a lot of content that can be downloaded for free has no advert type stuff in it that can be used as long as you tag the artist etc in your upload to YouTube - they often have examples of what is required on YouTube for you to use it. I have had some decent results in finding appropriate music this way to use it without getting the video taken down. You may also find a load of content on YouTube in dedicated channels that offer Royalty Free music as long as you tag the artist or sign up to their news letters etc too.