Ok, if you're starting to get depressed then that's not good. Just take one step at a time.
First step, make yourself feel a bit better. Now, for me, when i'm upset or stressed i watch an episode (or 2) of Friends or Red Dwarf. That usually makes me feel much better.
Second step, do something with your day. Have you got all your stationary organised for when your course starts? Maybe you could just go into the city and buy some paper/files/pens etc. Or you could buy some sheets of colour card and glitter glue pens and make some posters for your room when you get back - i did this, by printing off a load of photos of my friends, and now i have 3 really cool posters in my room that i've kept for 3 years.
Third step, a cuppa tea solves everything. Is there someone in your house that you've spoken to a bit more than others? Perhaps you could knock on their door and see if they fancy a cup of tea and biscuits. If they're not in, try someone else's door. You are NOT the only person feeling like this. Most people get blues and homesick at some point in Fresher's week. Obviously it's not easy to just knock on someone's door, but they'll respect you for it. If you don't make an effort with them, then why should they make an effort with you. Making an effort with one person has got to be less scary than making an effort with a big group, so please just try it. If they say they don't want a cuppa tea though don't be disheartened, they might just be busy. When you're chatting to whoever it is i think you should ask them how they're feeling, if they're a bit homesick etc. Because, unless they're inhuman, they'll probably be a little scared/worried about something. "Are you looking forward to your course?" "I'm a bit nervous about walking in that first day to a lecture, are you?" etc.
If no one is in then it gets a bit trickier. But if that's the case, and you've tried the other steps feel free to PM me. Do you like rugby? I don't spose you do, but the England game starts at 3 so you could always go to a pub and watch that? Even if you don't like rugby it'd get you out of your room.
Also, leave your door open. Obviously not if you're an emotional mess. But at all other times, keep your door open if you're in your room. And if anyone walks past, say "Hey, how are things?"
Finally, i'm sure you're aware that most of the people are probably not as confident as they seem. In fresher's week i found that i just had to put on a confident act. Perhaps for 1 or 2 days, just until i'd found a couple of people to talk to. Whenever you meet someone, wherever you meet them, exchange numbers and then follow it up, arrange to meet for lunch or something. Confidence is an act, and it's fairly easy to fake - especially in Fresher's where no one knows anybody. 3 days is NOT long enough to make life long friends with people, and everyone is open to meeting people.
This whole thing probably sounds patronising and it's really long, so sorry. The problem is, that if you don't do it now..it'll get even harder. Take deep breaths and go for it. Just DON'T sit in your room all day alone. It'll make you feel worse.