Hi,
without using anytime before GPOs, i suggest you built yourself a test system to learn how to handle them. Or at least use a Test OU with test computer and test user account.
As a starting point, do not modify the Default domain policy or Default Domain controller policy. Leave them always at the default settings and configure and link additional GPOs when you will change something on that level. In case something fails you can delete the new GPOs and are back.
See here for a starting point:
Windows Server Group Policy Home
Also DNS has to be configured and must run correct that GPOs are applied correct. DNS is the most important role within AD. Make sure that all servers/clients use only the domain DNS server on the NIC and not your ISPs.
Also see this White paper:
White Paper: Administering Group Policy by Using the Group Policy Management Console
Here some additional about Group p[olicy preferences and starter GPOs (check also the included links):
Group Policy related changes in Windows Server 2008 - Part 1: What are Starter GPOs?