Technically, most of the items in your home are probably owned by your parents except for your personal items or items you have purchased yourself. If you take your clothing, mementos, jewelry, personal items, etc. (i.e. things in your room), it most likely will not be a problem. The only option your parents would have would be to call the police or to sue you and in those cases, no one is going to fault you or do anything to you for taking those types of things.
However, if you start taking other items such as furniture, it may become an issue. I assume that this is not something you can discuss with your parents since you are leaving at 18? That would be the best option. I am sorry you feel that you need to take this action.
As far as personal items: take anything that is your clothing size and gender and generally has been worn by you in the past. Take anything that has been gifted to you by anyone ever. Take anything you bought with your own money and take anything that has your name on it. Don't take a car unless it is titled to you and only you.