Most new graduates arrive at Christ Church a week or so before Michaelmas Term begins, in early October. There will be plenty of events arranged by both the college and the GCR around this time, and they should give everybody a chance to meet new people and learn names and faces.
It is easy to feel isolated if the requirements of your course, or some other reasons, mean that you have to arrive in Oxford earlier. If you do have to be in Oxford earlier than the other freshers, make sure you get in touch with the GCR president, so that you can have a tour of the common room facilities, and be introduced to GCR life. Feel free to come up to the common room and introduce yourself; the people there may very well be new arrivals also, and you'll find that most people will be happy to meet newcomers.
The vacation dining service will be running, and is an good way to meet new people, so sign yourself up.