I just returned 2 days ago from 9 weeks backpacking Europe, so I am happy to help you out!
Lockers: Most hostels have them; some do, some don't. Sometimes your pack will fit inside, sometimes it won't, sometimes hostels only have small lockers for essentials like passports, etc at the front desk. Regardless, if a hostel doesn't have a locker or it's not big enough, I wouldn't stress too much. Simply do two things: Carry all your valuables in your daypack and/or moneybelt (passport, cash/credit cards, iPod, etc) and take them with you while you're outside the hostel, and realize that in your pack you likely have replaceable things - besides, no one wants your dirty clothes, you know? Adopt that mentality and you'll be fine. I had nothing stolen from me in the 30ish hostels I stayed in, but with that being said, it's all about common sense.
Hostel ratings: Take them with a grain of salt. Trust me. People sometimes leave ridiculous comments - "the showers were cold!" Think about it - that person probably showered at 10am when everyone else was, so of course there was no hot water left. Read through them, but if it seems good just go with it. Every hostel is different, some are great, some aren't so great but hey, at least you had a bed for the night.
Sheets: Definitely bring a sleep sheet/sleep sack. I wouldn't haul around actual sheets - too heavy. Buy or make one and you'll be golden, especially when some hostel sheets look a bit... suspect.
Laptop: Eh, your choice. For 30 days abroad, I don't think it's necessary. They are simply too heavy most of the time and you definitely risk it being stolen (odds are you won't want to carry it whe you're in a hostel without a hostel, you know?) Yet they are definitely convenient. I would just rely on internet cafes for how long you'll be there.
Feel free to PM me if you have more questions, happy to help out!