do you want a fun-loving flat, quiet study base or sociable hostel?
Flatting
It all sounds pretty cool and romantic when you're at secondary school -- living in a house (that you can decorate however you want!) with a group of friends the same age as you. And it can be an awesome experience. It's a great way to learn about yourself, like, are you a clean freak or a slob? Do you have to do the dishes straight after dinner or can you leave them in the sink for a few days? Do you wait until you've used every last piece of clothing until you decide to wash something?
Choosing who you flat with is a decision that shouldn't be made lightly. You will be living with these people, and quickly learning all their daily habits (good and bad). Living with existing friends is great because you already know each other and it provides plenty of hanging-out time. Just make sure you spend some time apart as well. The other option is to move in with a bunch of 'randoms' -- people you didn't know before. This can be an opportunity to meet new people who may be completely different from you, and be introduced to new circles of friends.
Don't be afraid to ask questions when looking for a flat or flatmates. If you prefer quiet evenings, then living with someone who likes to party all the time won't work. If you like to have a few drinks with friends, then moving into a non-alcoholic flat will bring up some problems.
Hostel
It can be a nerve-wracking experience -- moving into a hostel full of people the same age as you, some of whom appear to be drinking 24/7.