Do you want the site to be accessed in your LAN only? or public?
If on LAN only, first thing to do is to create website. On the ispconfig 3 just click on a tab at the top named SITES. Then on the left click WEBSITE then click CREATE WEBSITE. On the Domain name enter e.g example.com then enable other features like php,cgi e.t.c
Then click create website.
after u r done with website creation if u dont have nameserver to resolve name to ip, you have to add that website in ur host file. e.g in windowz XP windows\system32\drivers\etc\host file.
this means ur machine will resolve name to ipaddress.
Keep in mind you will have to do this to every pc for it to access the site.
Then open ur browser and point to website name u've created in ispconfig. BRAVO u r done.
Remember you cannot access ur site using ipaddress of the server.
If you want ur site to be accessed worldwide then ur server must be assigned static public ip address and ur registered domain name must point to ur server ip address.
DNS settings can be configured in ISPConfig if you want. but if ur DNS is hosted elserwhere just make sure it points to ur server ip address.
hope this gives you a good starting point.
i've not covered everything coz ispconfig has soo many features and i wrote this using my phone.
wish you best of luck.