When you have ISPConfig 3 installed, you want to setup a website. This tutorial should help with that.

On your domain registar side:
1. you have ordered a domain name
2. you have registered new custom nameservers using your server IP in these (usually people name NSs like this: ns1.yourdomain.tld, ns2.yourdomain.tld)
3. you have changed your domain nameserver to ones registered in step 2
4. your domain is now resolving to your ISPConfig server and looking up for a DNS response

On ISPConfig server:
5. click DNS icon, Add new DNS zone wizard
6. enter values & submit:
Domain: yourdomain.tld
IP address: your server address (one in the ISPConfig site address bar)
NS1: ns1.yourdomain.tld (ns1, ns2 is optional, depends on what you used in step 2)
NS2: ns2.yourdomain.tld
7. click Sites icon, Add new site
8. enter values & submit:
Domain: yourdomain.tld
Let's Encrypt SSL: tick (if you want https://)
PHP: PHP-FPM (or other available)

Is Named working and running? service named restart

After some minutes, check https://dnschecker.org/#A/yourdomain.com & https://intodns.com