The first place you should visit when you arrive at Christ Church is the Porters' Lodge. You'll find the Porters' Lodge in the arch of Tom Gate, which is the main entrance to Christ Church, on St Aldates. If you will be staying in college accommodation, then you should be able to get a key for your room at the lodge, along with a Tom Gate (wicket) key, and the code for entrance to St Aldates Quad, if you're staying there.
A key to the GCR has to be collected individually by every graduate from the Secretary to the Tutor for Graduates, Melanie Radburn (Tom 8, first floor).
The GCR key opens the main door to the GCR, but also opens the dining, computer, and study rooms, and the GCR kitchen. All these rooms are on the first and second floors of Tom 7. There is a key deposit, which is paid by battels, so try to hang on to your key.