Welcome to the Apple Support Communities!
I understand you have been able to use the Messages app on your new iPhone X, but you have not been able to use the app on your iPad. Can you provide us with a little more information about your iPad? What model is it? What version of iOS? Are you signed in with the same Apple ID on both devices? What happens when you try to use the Messages app on your iPad?
The resources below will provide the basic settings and information for using the Messages app on your iPad:
iPad User Guide - Set up iMessage and SMS/MMS
Add or remove your phone number in Messages or FaceTime
If you get an error when trying to activate iMessage or FaceTime
Use Messages on your iPhone, iPad, or iPod touch
After reviewing this information, if you are still unable to use Messages on your iPad, please reply to this thread with any additional information you can provide and we'll do our best to help!
Enjoy your day!