Games operator and retail stand clerk at a kiddie amusement park when I was 16.
My advice: treat EVERY customer as though they’re a secret shopper who might write a review of you. (Guess who got reviewed while having a bad day and wasn’t asked to come back and work the next year?)
Hehe, judging by my stints crab processing, I agree with you that it’s probably the tint of the rear-view mirror making it look so pretty. 
Aside from yardwork/babysitting/paper delivery, my first time-card job was…babysitting. At a health club. :smack: Soon after that I got a bagger job at a grocery store; the advice given upthread is about all you can use; everything else is learning by doing. 
And good on her; I hope she enjoys it a little bit, at least. If she’s there after a year, let her know she can move on and look for something else; she’s not stuck there.
Paid?
Book store, Waldenbooks, many decades ago. Loved it. $2/hr, mucho dinero. Spent most of it on books, purchased at a discount. They put me in charge of the Science Fiction section.
Worked some construction in college.
Oh. I never added my advice to the new worker.
I’ll just echo others: Be on time
Slackers that arrive late and leave early are a real irritant with me. Being on time is the easiest part of the whole job. If you can’t get that part right…
My first continuing job was as assistant to the director at a funeral parlor. My advice? Remember that in any job, there will be laughs, if you only look for them.
My first real, had-to-apply-for-and-received-a-paycheck* job was as a busboy for Planet Hollywood Chicago, during my sophomore year in university. Boy it was an eye opener. Shitty pay, but fun! The obvious advice - show up on time, work hard, be the guy/gal that everyone likes.
*I’ve had other jobs before, but they were all stipend-funded research/work-study positions.
I worked at a Rax roast beef for a whopping $2.85 an hour. Somehow, Rax managed to decide that minors didn’t have to be paid minimum wage; they thought sub-minimum would suffice. I stayed there for three months until a gig as a page at the local county library opened up. That was a county gig, so I got a raise to $3.58, above minimum, which was $3.35 at the time. I was rollin’ in the dough! After a year and a half, I got a raise to something like $4.11 and was one of the highest paid high school seniors in my class. LOL I worked there for two years and saved 90% of what I made. ALL of it was completely wiped out after paying for just one quarter of tuition. 