Question:
"Does anyone have any advice regarding hostels."

As long as you know generally where you're going, you should book hostels a few days in advance, since the better ones tend to fill quickly. If you are visiting London, Brussels, Amsterdam, and Berlin in that order, book the Brussels hostel on your first day in London, and book the Amsterdam hostel on your first day in Brussels, etc. I've never had a problem getting into *some* hostel in a city, but the most famous and best located ones often fill up quickly. At most there's a credit card reservation charge of around 1-2 euros per night for reserving a room.

Even if you want to party, you might not want to stay in a party hostel. Many of them are loud at all hours of the night, either because of bars on the premesis or because of being in a noisy area.

Hostels normally have from 4-10 people to a room. If there are 8-10 people, it will often be noisy because people are always coming and going. Absolutely bring a lock - thefts from bags in hostels are unfortunately common and you shouldn't leave your stuff lying around.

Answer: Some hostels that I particularly recommend:

London: St. Christopher's Camden - it's in a really interesting area, and it's one of the cheapest places to stay in the entire city.

Liverpool: Embassie

Rome: Colors, short walk from the Vatican

Amsterdam: Globe, right in the Red Light District

Ios: Francesco's

Brussels: Van Gogh Hostel

Salzburg: YoHo