Honey you are so young. I think you need to tell the truth. It’s unlikely (hopefully) that she will actually kick you out. If she does try, that is illegal (in the US if that is where you are) to do so. I know it’s hard and scary but it’s the right thing to do. 13 is so, so young and for whatever reason you made this mistake. Hopefully your mother is not abusive and you will get past this.