First, If you wan't to block Youtube's ads in the app, you will need a rooted device and a new framework for your phone.
Xposed is the ultimate app for making modifications to your android phone. If you can get Xposed installed and have it working, download an app called "Youtube Adaway".
Youtube Adaway is an Xposed module that works like a charm.
If you wan't to download music off of Youtube, you will need to use a website like mp3fiber.com.
Simply copy and paste the URL of the video/song you want to download, and it will get your mp3.