How can I do backup of Active Directory windows server 2008?

You have to do either Full Server backup or SystemState Backup

Install Windows Server Backup Feature throgh Server Manager.
Server Manager - Features - Add Features - Windows Server Backup Features .
After the Installation, Administrative Tools - Windows Server Backup - Then Click Backup Once and Follow the Wizard To take Active Directory Backup You Can select 'System State' Alternatively you can use 'wbadmin' command Wbadmin start systemstatebackup

Install Windows Server Backup Feature throgh Server Manager.
Server Manager - Features - Add Features - Windows Server Backup Features .
After the Installation, Administrative Tools - Windows Server Backup - Then Click Backup Once and Follow the Wizard To take Active Directory Backup You Can select 'System State' Alternatively you can use 'wbadmin' command

On a writeable domain controller, a) create a folder ‘D:\adbackup’ on a partition other than the boot partition. 
Next, go to the command console (cmd) and perform the following steps b) Enter ntdsutil (brings you to ntds: command mode) c) At the ntds prompt, enter activate instance ntds (ntds will be activated) d) Next, enter ifm (brings you to ifm: command mode) e) Enter the command create full d:\adbackup

System state backup is sufficeient for backing up Active Directory.

create a system state backup with Windows backup feature

Backup procedures have changed in Windows Server 2008, as compared to previous versions of Windows Server. A new backup tool, Windows Server Backup, replaces NtBackup as the tool that you use to back up AD DS. You cannot use Ntbackup to back up servers running Windows Server 2008.

In Windows Server 2008, you can perform three types of backup:

• System state backup, which includes all the files that are required to recover AD DS
• Critical-volumes backup, which includes all the volumes that contain system state files
• Full server backup, which includes all volumes on the server

You can use the Windows Server Backup graphical user interface (GUI) to perform critical-volumes backups and full server backups. You can use the Windows Server Backup command-line tool, Wbadmin.exe, to perform all types of backup, including system state backup.