DO doesn't offer domain registration. You need to buy your domain name from your preferred registrar and point it to your server by changing the nameserves of your domain to this:

NS1.DIGITALOCEAN.COM
NS2.DIGITALOCEAN.COM
NS3.DIGITALOCEAN.COM

And then goto your control panel on DO, navigate to DNS and click on "ADD DOMAIN" at the top.
Select your droplet, enter your domain name and voila.

Hello,
I have installed zpanel but i can not understand how configure NS1 and NS2 record, like i want to create NS1.mydomain.com, suppose if i ping NS1.mydomain.com so it will reply my zpanel server ip. and want to configure my domain control panel NS record so that i can recive email.

Check out https://www.digitalocean.com/community/articles/how-to-create-vanity-or-branded-nameservers-with-digitalocean-cloud-servers (Branded Nameservers).