Pharamacy is like checkouts but you also need to know a little about medicines because you're behind the pharmacy counter. There is a course you need to complete, but it's one you do while working and isn't too challenging from what I've heard.
Interviews vary from store to store but don't get hung up on it, they're interviewing for part time supermarket staff, not the next apprentice.
The pay for under 18's was around £4.70 last I heard it might have gone up a little since but probs only 10/20p.
Trainings fine, like I say the job is not overly challenging so training doesn't exactly need to be extensive. In the case of the pharmacy job it's a national training scheme required of everyone who works behind pharmacy counters so it's rigorous enough for what you need.
And the last one is something only the store can tell you. I'd imagine they have set hours they'll need to cover (else why would they be hiring?) but they're probably taking on more than one person in which case there might be some flexibility in which person covers which hours. You will given a contract though, the hours aren't flexible week by week (well you can negociate changes, but you do have set hours).