Do you have to find out/ prove something to get a PhD? I went on a trip to Oxford with my school, and I asked a lady whether or not you have to go and discover something in a PhD? She said no, and it's more research. But I read up somewhere that you do have to discover something to get a PhD.

I just want to know precisely how a PhD works.

Will some subjects- like Maths or Computer Science- require you to go out and prove something, and are others- like English- just require the student to do extensive research?

Note: by getting a PhD, I don'y just mean anyone getting it, I do mean someone who's actually studying for one.
Getting a PhD almost always requires one to have some original contribution to your field. That result doesn't always have to be an earth shattering proof of something completely original. Your work can build upon existing work, or draw connections between a variety of different topics that no one has noticed yet, or even just suggest that there is a problem that nobody has considered yet. The point is that there has to be something unique, and original that counts as your contribution to the body of knowledge. That isn't easy to do certainly, but it isn't impossible for normal people to accomplish through lots of time and hard work either.