What should I do if someone hacked into my Apple ID?

Last night, someone went into my Apple ID. This person sent a plenty of abusive messages to my contacts. I know I didn't do it, because I was just busy watching videos while that person was sending messages. So I have blocked all my contacts to keep low from this person. Well I used to a iPod touch and I lost it last year in my school. After when I lost it, someone must have found it and went inside that account. So is there anyway I can do?

Added (1). I own a iPhone 6S btw

The only way they could do it is if you had such a stupid password that they could guess it, or you were dumb enough to leave a password laying around somewhere or leave your phone lay around where they could get on it.

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