If you are travelling for long time, get a good quality backpack, which fits you comfortably and avoid to pack it with too much stuff - an overstuffed backpack can be very heavy to carry and not very comfortable in summer when it is hot. Go for comfortable shoes and keep cool shoes for your nights out only.
Hostels most often provide you with sheets (sometimes at an extra cost therefore you should bring your own, just in case. Some hostels might not allow sleeping bags for hygienic reasons (you might have been sleeping anywhere with your sleeping bag, and hostelling is not the same thing as camping).
Some hostels will provide you with a so-called sleep-sheet, which are two sheets sewn together so that to form a sort of sack; this way the bed stays clean. It is not as common as it used to be years ago.
You should also bring your own toiletries and towels, since only some hostels provide them, usually at extra charges.