Help with copyright for videos?

[Megan]

Hi I have an Instagram account called horsevideos1 and what I do is I take YouTube videos and add music and crop and split them till they look veery different from the original video. Today I posted a video and tagged the person in the videos insta account and they replied. She said that what I was doing is illegal (ya duh) and she wanted me to take it down. But the video wasn't on her YouTube page it was on another page and there was no official indication that It was copyrighted. I am very new to the idea of copyright and I was wondering what I would need to do to repost videos legally