Hi Jucao, iControl is a set of web services exposed on the F5 devices. Think of it as a programmatic interface to the admin GUI. In fact, it uses the same management interface and authorization as the GUI.
The API exists on the device so there is nothing you need to do to enable it. What you do need to do is to pick a client toolkit you would like to use. The client toolkit will interface with your client program (C, VB, Java, Python, Perl, PowerShell, etc) and allow you to access the methods without worrying about the format of the wire-level content of the API.
I'd recommend looking at the iControl wiki homepage at
https://devcentral.f5.com/wiki/iControl.HomePage.ashx
for background info as well as some quickstart guides or the various languages. You can find the client packages in the DevCentral Labs projects
https://devcentral.f5.com/Download/Labs/tabid/1082206/Default.aspx
Hope this helps getting you going.
-Joe