To use your domain or subdomain with Masto.host, you need access to your domain DNS settings.
In most cases, and if you have not changed it, your domain registrar is the company that also hosts your domain DNS. So, it would be best to look in your domain registrar or your DNS hosting service for a page called DNS Settings/DNS Zone Editor or something similar.
Note: this is not Name Servers. You don't need to change Name Servers to point your domain/subdomain to Masto.host infrastructure.
For APEX/root domains
You only need to change or add an A record to point to Masto.host.
A note regarding IPv6 on APEX domains.
For subdomains
You must only add a CNAME record to point to Masto.host.
Instructions for your installation
You can find specific DNS instructions for your installation on your my.masto.host account.