Someone else used my Apple ID to make a purchase what do i do ?

Did you get an email message indicating that?

Did you verify that the message was actually from Apple, and not one of the scams seeking to get you to log into a bogus Apple login page, so that the folks can actually get your password, and can actually then access your Apple ID?

Did you verify the reported purchase against your Apple ID billing information?

As for your question, if your Apple ID and password was actually exposed, you'll need to reset that password pronto, and pretty much every other password including the passwords on your email accounts as access to those can be used to reset many other passwords you might have, if the email messages don't also include passwords in the mail messages.

Do you have two-factor authentication enabled? If not, go turn that on right now.

Read this: Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams - Apple Support