Decide on a domain/subdomain to use
You can use a domain you own or a subdomain you own to install Mastodon.
This means that if you own the domain example.com, Mastodon can be installed there, and it can also be installed on an example.com subdomain (like social.example.com, mastodon.example.com or anythingelse.example.com).
The domain/subdomain you choose must not be used for something else. If you already use your domain to host a website, install Mastodon on the subdomain, or the website will stop working.
Alternatively, you can use a free Masto.host subdomain. This will be a subdomain that ends in masto.host (like yourname.masto.host, somethingyoulike.masto.host or somethingelse.masto.host).
Notice that when choosing a free Masto.host subdomain, you lose portability. This means that in the future, you cannot migrate your Mastodon server to a different hosting solution because Mastodon doesn't support domain name changes and Masto.host subdomains can only be in use on the Masto.host infrastructure.
Do you want to use a subdomain for installation and a domain for usernames?
Please read: Steps to use a subdomain for installation and a domain for usernames
Are you migrating your server from self-hosting or another hosting solution?
Please read: Migrating server to Masto.host
A way to pay for the service subscription
Please read: Payment Methods
Access to your domain DNS
In most cases, and if you have yet to change that, 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.
If you're using a free Masto.host subdomain, then this step is not required.
After you choose a plan and follow the signup steps, Mastodon will be installed, hosted and all the tech stack will be maintained for you.
Feel free to reach out firstname.lastname@example.org if you have any questions.