University open day dates seem to have mostly come out this year and can be found on their websites. Lots in June/July and then another wave in the Autumn.
I think it's quite hard to know where to start, but broadly I think you're trying to work out what subject they'd like to do, what sort of place they'd like it to be (big city, city campus, rural-ish campus, smaller town etc), how far from home, how good a fit their likely grades are with the places they like etc, and go from there. Budget is also something to factor in - there is a massive discrepancy between accommodation and living costs around the country. You choose up to five course to apply for. Once you have your offers you pick two (favourite = firm, second favourite that you'd be happy to go to = insurance). Ideally you want a range of grade options in that five, so you can have an insurance choice that has lower grade options.
DD is planning to apply for Medicine, which (like Oxbridge, dentistry and vet medicine) has October deadline rather than January. So, choosing a course has been the easy bit (getting on it might not be so much!). She is trying to get a good chunk of her research done this summer and we have booked out several weekends for open days already. She definitely wants a medium-large city, and we've ruled out anything north of Leeds due to distance from home (also ruled out the couple of places closest to us as she wants to move further away). I'm lukewarm on London due to the costs, but that's probably where she most sees herself at the moment.
As @mumonthehill says above, school/college should provide a lot of support and will know the process well. Exciting times!