Bunch of questions...! Hostel, photos...

Im going to Japan soon, Ive been there before, but I went with friends. This time, Im goin alone
In Tokyo, are there any cheap hostels? Do they give you a key, so i dont have to drag my luggage and passport with me?
Speaking of which, do I bring my passport with me in my bag? All the time? Or trust it in my hotel/hostel room.
Photos. I've read on here that you can, just dont do anything bad and use common sense. Ive seen videos taken in Donki stores, how do you do that? Just go untill someone says no?
Thanks!!

EDIT: Also, a friend wants me to pick her up a sailor uniform. I heard Cosmate, Jupiter and Donki are good spots...?

I assume you have abandoned this thread as no one has responded... Yes you get a key, and there are usually lockers to lock your bags while you leave. Whether you carry your passport is up to you... You technically should at all times, but I haven't always done that. Actually in tokyo I only carried it 1/3 of the time. Don Quixote is definitely a good place for costumes. Go to hostelworld.com, each hostel has extensive info about their amenities.