hacked

Someone has hacked my apple ID recently... a notice came up on my phone that two separate mac computers accessed my apple account... i then received a bunch of text messages that included chinese characters and emoticons of hearts, spades and clubs i believe... i am unsure what to do because i got a notification today as well stating that this computer that hacked my account is now using it for iMessage?

PLEASE HELP!

thanks so much

L.