Q: How can I prevent someone from using my apple ID and telephone number to FaceTime on their iPad?

The first question is: how did they get your ID and password? Did you give anyone access?

Don't give your credentials to unknown users.

If you want to change your password

If you know your Apple ID password and want to change it, follow these steps:

1. Sign in to your Apple ID account page.
   
2. In the Security section, click Change Password.
3. Enter your current password, then enter a new password and confirm the new password.
4. Click Change Password.
5. Sign in with your new Apple ID password to access Apple features and services.

Learn more about how to change your Apple ID and update other account information.

from Change your Apple ID password - Apple Support

Change the password to your Apple ID as soon as possible and don't let anyone else know what the new password is.