Selecting the song is the real challenge. If you are making Animation Music Video for fun or for educational project then you can take any song but remember to give credits to the composer or the singer of the song.
If you want to make it for professional purpose, you have to take official/ written permissions or it is better to choose royalty free music. There are many online sites available where you can choose between wide range of music’s and songs that are Royalty Free.
There are different genres of songs like hard rock, soft romantic, very fast and intense, light hearted, very slow and sentimental, chase, rock n roll, etc. everyone has different taste in music however, you can select your own song.
As a beginner, I would advise you to start with a simple song. Which has a comparatively consistent beat and a simple structure in terms of music composition so animation sync can be easier.
If you are a singer or musician, you can create your own piece of music and animate to it. This will be easier because while composing song or music, you can think of appropriate visuals for it.
Select the right song that has lots of scope for animation and creativity. It is necessary to fall in love with the finalized song so you can enjoy the journey of making animation for the music video.