How do I add FaceTime to my iPad.

FaceTime is part of the iOS. You cannot add it. The fact it is missing could be for any number of reasons. The device may be from an area where FaceTime is prohibited, it could be turned off in Restrictions, or you could have a device that is too old for FaceTime.

Since you do not indicate the answers to any of those questions, you would need to provide more information for anyone to help you.

There are many various models of "iPad"...you'll need to identify your model to get some meaningful replies here.