While many hostels provide lockers for tenant use, you should always look when booking to see the extent of the locker. Do you need to bring your own lock? Will the locker be large enough to hold what you need locked up? Is there space for your things besides the locker, or is that it? This will help you decide how to pack, what to bring, and how to store it.
Some hostels don’t have lockers but common space for luggage. In these cases, many travelers don’t feel comfortable leaving important documents/valuables unlocked. In these cases, I suggest having a separate part of your luggage for valuables – cameras, batteries, phone, cash, cards, passport – and using a small padded case to store these things. Then you can also use that as a pillow to sleep with your valuable so that you don’t need to worry about night-scavengers.
You also want to look before booking a hostel to see if they provided towels, bed linens, etc. And what kind of kitchen accoutrements are provided.