How can I pick my bathroom door lock?

I just tried to open the door to my bathroom and somehow it is locked? I keep it closed so my cats dont go in the bathtub now I cant get in! Cheap, rundown, ghetto a** apartment, the stupid b*tch landlord just said "well you must have locked it yourself" Are you kidding me? Why would anyone do that?? Anyways I need to know how to pick a lock, its an old door knob kinda triangular in shape with a slit in the middle, I tried turning the slit thing with a knife but it didnt work... help?