I think my iCloud account has been hacked

My son just tried to FT me using my iCloud account. Before I could pick it up on my iPad, the call disconnected. He called again seconds later and asked what just happened. He then explained that on his end, that first FT call was picked up by a 12-16 year old white male he'd never seen before! My son asked who he was and the kid responded with the name part of my iCloud account! My son disconnected the call. This happened again about 10 min. later when he tried to call me again. Same kid!

What do I do now? Is all of my stuff at risk and how to I change what needs changing to make it secure again? This is very unsettling...and creepy!