Firstly, it is great that she knows what she wants to do and is coming at this early enough.
Here is the admissions page for NatSci at Cambridge, (this is a marathon, not a sprint) and MN is here to help you with any questions you have. They want A A A in 3 science/maths options, details are on the admissions page and you have to sit an entrance exam too.
www.undergraduate.study.cam.ac.uk/courses/natural-sciences
Here is the information on "supra" or Super-curriculars from Cambridge Christ's college.
www.christs.cam.ac.uk/sites/www.christs.cam.ac.uk/files/inline-files/Supra%20Curricular%20Enrichment%20Guide%202018-19_0.pdf
Basically, for any top uni, Oxbridge or RG etc some ask for A A A or A* AA and pretty much every student applying has those grades. More students apply than there are places so it means it comes down to admissions tests and demonstrating your love of a subject. One of the admissions people at Cambridge said, we want a student to want to get out of bed, walk through the pouring rain to come to a lecture and want to do the work we set.
It isn't enough to say how much you like a subject, your DD needs to show how much she likes it. What has she read or seen on her chosen subject? What courses has she taken? This then leads onto what has she learned from this? Did something specific catch her interest, where did this lead her, more from the same area or a look into something else.
In the summer of year 12 (usually) she can look to attend a summer school or a masterclass in her subject. Look at Sutton Trust. If she is the first person in the family to look at university then this gives her a leg up the criteria to get onto these summer courses.