What to do when someone take your private mail and opened it.?

I live in a first floor apartment and my neighbor of the apartment b took my social security card that was mailed to me. she took it and opened the envelope and was untill the next day when she gave it to me. She only told me that didnt know who was it. Maybe she doesnt know my name because I just moved there, but still the envelope has my address correct with the Apt A. I pissed off about it. Now I'm worry about if she took my ss#. I'm thinking make a police report just in case of she want to use it. somebody tell me what can I do. or what do you do if this happen to you.