How do I make an appointment at an Apple Store?

First, an answer to the question in the title of your post; you can find and make an appointment with your local Apple Store or Apple Authorised Service Provider (AASP) here:

Genius Bar Reservation and Apple Support Options - Apple

Turning to the supplementary questions within the body of your post, to be able to synchronise your iPad and iPhone via iCloud, both devices must be signed-in to iCloud with the same AppleID.

To send and receive SMS/MMS messages (green bubbles) from your iPad, assuming that both devices are using the same AppleID, you must explicitly enable Text Message Forwarding for your 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

You need to be logged into the same Apple ID. You can then enable text message forwarding and messages in iCloud to sync the devices.