All the best for your first solo travel. I have traveled alone to many places but I haven't stayed in a hostel yet. But I have met fellow travelers who have done so. In fact once in Brazil I met a guy who only stays in hostels because he want to meet and mingle to different peoples including the natives. he says that it gives a totally different experience to travelling. I wanted to try it once but honestly when the time for booking accomodation arrives I opt for a hotel or B&B.
I wish you good luck in your travel
I have been travelling solo many times and I love hostels (I like them much more than hotels and Couchsurfiung), cause there are places where you can easily meet other travellers, exchange experiences together and sometimes even travel together for a while. I stayed in many of them and even worked in few of them
Well, still you need to watch your stuff :/
As for your questions:
1) I'm staying for 5 nights but I assume this would be the same bed every night?
In 98% of cases yes. Sometimes, when the hostel have many bookings and they don't have enough space they can ask you to move. However if in that case they will tell you about it before.
2) Do hostels usually have somewhere to lock possessions away or would I have to take my things with me each day?
Most of hostel has, but not all of them (especially the cheapest ones sometimes doesn't have lockers), so you should always ask about it before or check on the website.
3) (I know this will probably be hostel specific but...) do hostels usually provide bed linen and towels?
Yes they do