Hey, it's a a minefiled, isn't it?
At the risk of being too simple, here is how it works.
Your son applies through Ucas. There is a fee (about 30 quid from memory). The deadline is usually the end of Jan for the year you applying to. /some jjis, like Oxford and Cambridge have an early deadline). You get 5 choices.
Your son has to write a personal statement and get teacher references. He has to input his predicted grades and any other relevant qualifications (so for music, you might put the grades you achieved in say, piano or whatever). Once the Ucas form is compete you press submit.
The universities will start to make offers. Once the offers are in you firm a choice and insure a choice. The insurance choice is one that you are pretty damn sure you'll get the grades for, so if you don't make the grades for the firm, on results day you'll be certain of getting your insurance. If everything goes horribly wrong and you miss the grades for both offers, you go into clearing. Clearing is where unis advertise the courses that didn't fill up and you scramble to get on one of those courses. Lots of phoning round on results day.
So that's the application process.
To get to the stage where you put preferred unis down, you start by thinking about what subjects or careers interest you.
So, say your son is taking stem subjects and wants to study pharmacology, you look at all of the pharmacology courses. Then you check that your son has the right a levels. If he does, then you look at the typical offer. So say your son is predicted and A and 2Bs, you look at unis around that entry requirement. One should be aspirational (so 2As and a B), a couple should be around the same entry requirements as predicted grades and one a bit lower (say, 3 Bs).
By now you should have a decent list of unis. Then you start thinking about where those unis are. So, say you live in Manchester, does he want to live within a 2 hour radius, or is he keen to go all the way down south?
Some unis are campus unis, so a dedicated space where all of the buildings are with accommodation in or very near the same site. Or a city uni, where the buildings are dotted throughout the city and accommodation is too.
Some unis accommodation is more expensive than others.
We looked at some unis in the midlands. University of Birmingham is a campus uni. Coventry Uni is a city uni. Warwick is a bit of both, but students often live in Coventry and commute to Warwick (not that far). Some students live at home and drive to uni every day or get the train to uni every day.
I'd set aside some time over a weekend and do all of the uni/course research. Then book into some open days (go on the actual university website, they'll be advertised). You should be able to visit the department, talk to the staff who run the course as well as current students. You should be able to see the accommodation and other facilities, like the library, student Union and stuff.
There are some things that are different, depending on the course. Arts courses may ask for a portfolio or interview for example.
Hope this helps! I have been a bit general about some elements, but the gist is there!
Exciting times.