Year 9 is the perfect time to start preparing. The biggest piece of advice is to read - read as much as possible and as widely as possible: not just in the chosen subject area, but to take an interest in and around all sorts of things, from politics and the news to what’s happening in science and the arts more broadly. A well informed and inquiring mind across the board is a huge asset. In the chosen subject, read as much around the curriculum as possible. A scientist? Read New Scientist and other related periodicals. Read nonfiction books about science or popular science. Watch the Royal Institution Christmas lectures from the last few years. An arts subject? Read novels - loads of them.
Go to art galleries or museums related to the subject (or talks, events - doesn’t have to be expensive, we all have more than enough access to free knowledge and information from galleries and museums to online texts). Universities, museums, science foundations — all offer free events and workshops for families, young people and school kids. Theatre tickets for example can be got hold of really cheaply for teens.
Show your child how to access public libraries, and how to get hold of books and journals free in their subject. Make use of all the stuff that’s out there online (but don’t only focus on online information and reading). There are loads of ways of getting hold of secondhand DVDs of good films, plays, etc., from Amazon marketplace to car boots. Encourage intellectual questioning and discussion. Be willing to talk about current affairs and issues of the day, from AI and economics to international politics and recent history.
Oxbridge now offer access visits via schools from quite early on, and you can certainly just go and look around, visit open days and so on, even in year 9.
What you really don’t need: expensive Oxbridge preparation courses, tutoring, paid activities, or extra teaching of any kind.
The key at this stage really is just reading (and a bit of watching!)