In order to monetize your videos on YouTube, you need to use Royalty Free and Licensed music.
YouTube has done an awesome job stepping up free to use songs in their audio library. People have always asked me where I find my music for my channel. Here's a few I recommend to use as background songs for your videos!
You can find most of these songs at https://www.youtube.com/audiolibrary/music.
1. Spring In My Step- Silent Partner
2. Happy Clap Ukulele (Available on Songfreedom.com)
3. Feelin Good-Kevin MacLeod
4. Water Lily-The126ers
5. Going, Going, Gone-The 126ers
6. Someone Your Own Size-RW Smith
7. Show Your Love-Otis McDonald
8. Otis McMusic-Otis McDonald
9. Vacation Uke-ALBIS