I suggest you start by reading about System.DirectoryServices[^] namespace first. This namespace contains the DirectoryEntry[^] class that you will be using in accessing the properties of the user objects of your Active Directory. The following are articles that you can use to start coding using the above-mentioned class. They have code samples too.
Querying Active Directory using .NET classes and LDAP queries[^]
Enumerating Active Directory Objects using C#[^]

Once you get familiar with it, you may also want to read Syed''s article[^] about a wrapper API for AD services. It''s just another way(probably easier) of doing things. Good luck. :)