I'm at Bristol, and basically I don't think you have anything to worry about! I didn't know a single person going to Bristol either, and I think that's true of a lot of people. But here's a few tips:
* Apply for a hall rather than a student house. The hall bars are great for meeting new people. But don't panic if you do end up in a student house; you'll just have to make more of an effort.
* Don't stick to the first people you meet too much. This was my mistake. In the beginning I spent a little bit too much time with my flatmates and then I realised a few weeks in that we didn't actually have that much in common. The first few weeks will be very tiring, but make the effort to get out of your room and talk to as many people as possible. Don't stick to people in your flat or on your corridor. Talk to everyone.
* Talk to people on your course. This is one of the easiest ways to make friends. Just strike up a conversation with the person next to you, and after you've been chatting for a bit, ask them if they fancy going for a coffee. Try to do this with lots of people on your course, and make sure you introduce everyone you meet to each other. Sooner or later you'll build up a great network of course friends. This is important, because they're definitely people you'll be spending time with throughout your three years, and not just in your first year when you're in hall.
* Join a society. For the best social life, join a sports team or a music ensemble. Even though I have loads of friends on my course and in hall, the people I'm living with next year are people I know through the Symphony Orchestra.
* Get to know people who post in the Bristol subforum on TSR. Get people's MSN addresses and talk to them lots before you go. It might sound crazy, but I'm good friends with a lot of people at uni I met on TSR. I even arrived on my first day to find that I was sharing a flat with someone from TSR, and that another girl who I'd been talking to a lot was in the flat directly above me. You may not stay good friends with people you talk to on MSN, but it definitely makes it easier when you first get there, as you have a point of contact with someone.