Apple now offers two stage security verification. So :-

Go to the My Apple ID page
Select “Manage your Apple ID” and sign in
Select “Password and Security”
Under Two-Step Verification, select Get Started and follow the on-screen instructions

You will need to change your password and not use one you have used in the last year.

It a simple process and takes about 3 days to verify. So in future when you do stuff involving your Apple ID you will be sent a code to your phone which you then use to confirm it is you and not some stranger hacking your details.