How to calibrate a stud detector?

I need to anchor a tall bookshelf to the wall. I bought a studfinder but it beeps continuously when moved across the wall. The wall is between my apartment & the next apartment so is probably concrete or whatever? I don't know. When I try the stud detector on an inside wall like the one that separates the bedroom & living room, it detects correctly. So how can I tell where to put the L brackets? I don't want to just go into the drywall because that won't be strong enough to hold it. Thanks from a DIY girl.