Hello everyone, I want to put a lock on my bedroom door because I cant trust the girl I live with, Help please?

I live with a girl who has become a true crazed monster. It sucks she was not always like this but I hear mental illness runs rampant within our age group. Some time after I made it obvious to her that she could not live with me after our lease was up she began to threaten me and I don't trust this new person she has become. I feel that I need to find a strong lock for my bedroom door to protect my personal belongings.(I need a locking door knob that can be locked securely from the outside or maybe inside too, with a key) Can anyone relate? I really need your help on this because I am lost when it comes to what kind of locking door knob I need? what kinds are really good/strong and what kinds cant be unlocked from the outside (without my key) or picked easily. I really appreciate the help of anyone who even takes the time to read this~ yours truly