Some hostels have a set maximum duration for your stay, from three days to two weeks. If your time is up before you're ready to go, you can always move to another lodging for a night or two and then come back a day later and start over. Some hostels don't take reservations. Popular hostels in popular destinations can fill quickly, so it's always good to plan to arrive early in the morning to get a bed for the night. If you arrive late in the evening, you might find the hostel full.
Many hostels provide sheet rental for a couple dollars a night.
A good idea if you are particular about who might have slept on that bunk before you. It's also a good idea to have a travel pillow or pillowcase. The opposite sex. Some hostels, again, particularly in Europe, have co-ed rooms. If this makes you uncomfortable, just say so, and they'll place you in a room with others of the same gender.