How do I report my iPad stolen?

Apple doesn’t take reports of stolen devices or get involved in their recovery.

See this:

If your iPhone or iPad is lost or stolen - Apple Support

Start here:

Locate a device in Find My on iPad - Apple Support

Here is Apple’s advice for lost or stolen devices:

If your iPhone, iPad or iPod touch is lost or stolen – Apple Support

Use the Find My app to locate a lost or stolen device – Apple Support

If the Find My iPad feature was enabled prior to it being “misplaced”, then if active, you would stand a good chance of locating it. 

However, if the feature was not activated prior to being misplaced - or the iPad has been inactive or without a network connection for more than 24 hours, the last known location will not be available - and the opportunity to use technical means to recover the iPad may have been lost.