Just bought a new iPad and having trouble pairing my iPhone to my iPad, mostly in messages

You'll need to use the same Apple ID for both devices, and likely want to set up the new iPad and the existing iPhone to use the same iCloud settings for messages.

Here's the general setup for both, for iCloud Messages: Use Messages in iCloud - Apple Support

And for FaceTime: Use FaceTime with your iPhone, iPad, or iPod touch - Apple Support

And SMS: How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac - Apple Support

Are you having difficulties with SMS messages appearing on your iPhone - and not on your iPad - or are you having difficulty with all messages?

If you have an iPhone - and both iPad and iPhone are signed-in to iCloud with the same AppleID - to send SMS/MMS messages from your iPad you must explicitly enable Text Message Forwarding for your [new] iPad on your iPhone...

On your iPhone:

Settings > Messages > Text Message Forwarding - set to ON - and explicitly enable your iPad from the list of devices.

Also ensure that Messages in iCloud is enabled on both your iPhone and iPad:

Settings > [Your Name / AppleID] > Messages - set to ON

Just for clarity, the devices will not pair over bluetooth with each other, and it's not necessary for the messages to sync as explained above.