It's been a long time since I had a teen in the house. Love of course is the first key. Sometimes tough love. You are there to be their parent, not their friend. Teens need boundaries. Boundaries teach action and consequences in life. I was lucky with my teens, as they were never truly unruly.
I think it is important to state that discipline starts, or should start LONG before the teen years. If they know the rules, they are less likely to become unruly. At least that is what my experience has been. We also did foster care for two years. They look to you for guidance, no matter how hard they are trying to push you away. They also NEED the guidance and discipline.
A combination of love, and discipline can go a long way to a smooth transition. Keep communications open. Let them know they can talk to you, even if it is something you don't want to hear. Keep cool.
Best of luck to you.