Roommate needs help training unruly dog. My roommate just moved in and her dog has been very possessive and aggressive. The dog has already snapped at my dog twice and growled at me and small bites. 

I agreed to help her out since she doesnt have a lot of dog experience and her last roommates let the dog do what ever she wanted. Get in the trash, jump up on furniture, beg. The dog even got food like cookies and take out from previous roommate. 

I'm not sure how to start since the dog is so mean to me. Should I put her on a leash and make her stay with me and show her polite behaviours pay? 
How do I get her to stop being possessive over space, people, and toys and growling at my dog?

Any help is appreciated and I'll reply to comments as quickly as I can.