How can I get a list of users from active directory?

If you are new to Active Directory, I suggest you should understand how Active Directory stores data first.

Active Directory is actually a LDAP server. Objects stored in LDAP server are stored hierarchically. It's very similar to you store your files in your file system. That's why it got the name Directory server and Active Directory

The containers and objects on Active Directory can be specified by a distinguished name. The distinguished name is like this CN=SomeName,CN=SomeDirectory,DC=yourdomain,DC=com. Like a traditional relational database, you can run query against a LDAP server. It's called LDAP query.

There are a number of ways to run a LDAP query in .NET. You can use DirectorySearcher from System.DirectoryServices or SearchRequest from System.DirectoryServices.Protocol.

For your question, since you are asking to find user principal object specifically, I think the most intuitive way is to use PrincipalSearcher from System.DirectoryServices.AccountManagement. You can easily find a lot of different examples from google. Here is a sample that is doing exactly what you are asking for.

using (var context = new PrincipalContext(ContextType.Domain, "yourdomain.com"))
{
    using (var searcher = new PrincipalSearcher(new UserPrincipal(context)))
    {
        foreach (var result in searcher.FindAll())
        {
            DirectoryEntry de = result.GetUnderlyingObject() as DirectoryEntry;
            Console.WriteLine("First Name: " + de.Properties["givenName"].Value);
            Console.WriteLine("Last Name : " + de.Properties["sn"].Value);
            Console.WriteLine("SAM account name   : " + de.Properties["samAccountName"].Value);
            Console.WriteLine("User principal name: " + de.Properties["userPrincipalName"].Value);
            Console.WriteLine();
        }
    }
}
Console.ReadLine();

Note that on the AD user object, there are a number of attributes. In particular, givenName will give you the First Name and sn will give you the Last Name. About the user name. I think you meant the user logon name. Note that there are two logon names on AD user object. One is samAccountName, which is also known as pre-Windows 2000 user logon name. userPrincipalName is generally used after Windows 2000.