I just recieved a message on my iphone that my apple ID and phone number is being used on a new iPad. I dont have a new iPad and I did not recieve a text message that was just sent on my iPad

I have a MAC, iPad and iPhone. I received a message on my iPhone that my Apple ID and Phone number was being used one my new iPad. I do not have a new iPad and when my husband sent me a text message it didn't show up on my iPad. The text message from mu husband showed up one my MAC and iPhone. How do I fix this? How do I get whoever is using my information to stop?