Firstly, I would want to say well done in choosing to foster, and to care for a teenager. Many foster carers are nervous about caring for teens. I would suggest talking it over with your case worker, as their role is to support you and the child, and to help you work out strategies.
Teens need consequences and boundaries, but of course they do not like them especially. Children in foster care don't have a lot of say over their lives, where they live etc. They are quite disempowered, and I think this is especially the case for teens who are wrestling developmentally with issues that are additionally complex if they are in care. So it is unsurprising that they look to gain control wherever they can. So one strategy worth trying is to give options, room to negotiate. Don't choose to do this when they are upset though - emotional distress makes it much harder for them to hear you properly and to think clearly. You can do this without allowing the boundaries to be crossed without consequence. But it is worth remembering the point of boundaries - keeping them safe, teaching them appropriate behaviour etc. Sometimes it can be easy to slip into setting up battles that do not teach or help the young person, and become painful to enforce. If a natural consequence is not available, you could try giving a choice of appropriate consequences.
Another thing to consider is the capacity to regain lost privileges as a reward for appropriate behaviour. And try not to take away everything all at once. When teens in care feel that they have lost everything they can feel powerless, and be less motivated to try for change.
I hope these starting thoughts help. It is hard to be more specific without extra info. Definitely talk to your case worker about the difficulties, and perhaps consider family mediation too.