First establish membership and role provider. There is whole story about it. I will give a help here.
Here is link to SqlMembershipProvider (one of the options you can take):
http://msdn.microsoft.com/en-us/library/system.web.security.sqlmembershipprovider.aspx
Here is link to SqlRoleProvider (again only one of the options you can take)::
http://msdn.microsoft.com/en-us/library/system.web.security.sqlroleprovider.aspx
After you have established this you can limit user/role access on folder level. Put this code to web.config (inside configuration tag):
<location path="AdminPages">
<system.web>
<authorization>
<allow roles="Administrator"/>
<deny users="*"/>
</authorization>
</system.web>
</location>
<location path="UserPages">
<system.web>
<authorization>
<allow roles="Administrator,User"/>
<deny users="*"/>
</authorization>
</system.web>
</location>
Here is little explaination. Root folder "AdminPages" will be alowed only to users in role "Administrators". Root folder "UserPages" to users in role "Administrator" and "User". In both cases unknown users will not be allowed to access folders. This is all you need. Alternative to this is to create class that inherits from Page and then there handle page access... however I would not go that way.