Who to share with
With a bit of luck you will have been living in the University's halls of residence for a year before you have to make this decision. During that year you will have hopefully developed a network of friends that you can leverage to find the ideal group of people to share with. Even then it is a difficult decision. Just because you enjoy sharing a drink with someone in the university bar doesn't mean that you'll gel as housemates.
What generally happens is that one or two students take the initiative and put down a retainer or a holding deposit on a house to rent. This reserves the property and gives the initiators time to find the remaining members of the household. This can be done either by putting the word out through your network of friends or advertising on the university's notice boards, both real and electronic.
Now if it were me, I would rather be the one doing the choosing. That way you get to pick where you want to live and who you want to live with. When choosing your housemates start with a bit of self-analysis. Think about what type of people would fit in with your personality. Try and think about your actual self rather than your ideal self. You may like to think that you are a neat and tidy individual but if you don't always live up to this aspiration how will you feel when your unwashed dinner plate has "clean me" post-it note attached when you come down for breakfast. What I'm trying to say is be realistic.
Plan the process before you start. Try and build up the household gradually, one person at a time. When you have selected the first person invite them to join you when interviewing a possible second and so on. That way you give yourself the best chance of ending up with a group of like minded people.
One important decision you will need to make is the sex of the housemates. Exclusively male households (warning! Massive generalisations coming up) can become overly laddish and quickly degenerate in to a slum whereas a girls only property can become bitchy and go the same way. The biggest risk with a mixed sex house is that two of the housemates can become a couple, which is fine (if a bit annoying for the other members of the house) while it lasts but what happens when things turn sour? Will one of them have to leave or will they both stay and create an atmosphere.
Questions to ask when interviewing prospective housemates:
What is your attitude towards housework?
What time do you normally go to bed?
What type of music are you in to?
Are you sure you can afford your share of the rent?
Do you smoke