Im not a huge expert on this, can't remember much, but ds went to Oxbridge from a state school. There is a lot of help out there, don't worry too much.
The first thing is to be clear what courses she wants to apply to (can be different at different universities, but it's unusual for them not to be roughly the same subjects, maybe called something a bit different). Then what each university she's interested in requires for each course. Eg for ds, to apply for Computer Science at Cambridge he needed to be aiming for certain grades, plus he needed to do a specific extra exam.
The next thing is to understand the timescales abd deadlines - and I can't remember them! Ask your daughter what the deadlines for Oxbridge and other universities are - they are different.
Your daughter should be capable of finding all this out online but may want you to do it jointly.
Then find out about open days and visits, and see whether she/you both can go. Not absolutely essential but I would say not far off. If it's financially impossible, talk to the school - they should know about support grants for travel.
At that point you should both be in a better place.
Ds did have 4 sessions with a tutor to practice for the special exam, so that he understood what that would look like. That was all.
What Oxbridge are looking for is oeople who love their subject and people who like the teaching method. 1:1 or 2:1 coaching that they do is very intense and not everybody likes it. Their interviews are designed to work out who will enjoy that teaching and do well at it.
Hope thats some help.