How can i lock my room from the outside and still let myself in?

my sister keeps stealing my games that i have been collecting for years, and she knows how much i prize them to be. when im gone she gets in my room and takes them, then i find out later that shes playing them. my door knob is easy to unlock. on the out side all you have to do is get something flat and turn it, even your finger nail, so i dont see why theres even one there. so the next time im gone i wanted to lock my door to a way where only i know how to unlock it. i live in a apt so i cant change the knob or put a permanent lock on the door or anything. please help!