My grandma says I have a beautiful voice. How do I become a voice-over?

A beautiful voice is a nice start. But understanding how a text is built, seeing what the writer wants to say and how best to present it to the listener: that is the essence of the voice-over craft. On top of that, you need to invest in an acoustically suitable room, equipment, and the knowledge and skills to use them. And you need the skills of a freelancer, because you are becoming an entrepreneur.

If you think all of that suits you, a course (if you speak Dutch, for example at stemacteren.nl) could be a big step in the right direction.