OMG, Jr, sorry all these guys are giving you fatherly advice when you're just trying to explore your options....(although I have ta tell ya...they're right.) You think you'll never get old & bitter about your career as you'll be "different" and you'll "do it the right way," but regardless of where you end up--here, there, somewhere else--you'll eventually end up like the rest of us with the rude awakening of reality. (hey, man, I was gonna be a freakin' photo journalist for National Geographic--what the deuce happened?!? How did I end up at UPS???)
Anyway, in addition to all of the above threads, to answer your question, if you're 18 & pass the interviews, etc, you'd most likely start out as a package loader. You don't just go into being a driver as that's what everyone assumes, when they have what they think is the brilliant brainstorm of, "hey, why don't I try to get on at UPS as a driver?"
As a loader, if you put your dues in and absolutely work your butt off like you most likely have never done before or thought you were capable of doing, you may have a shot at becoming a driver, again though it depends on the factors like what Pickup mentions above. Loading packages is a very, very hard job, however, that being said, I wish when I was struggling through college working at the Waffle House (seriously) that I'd known about the loading position. The pay is pretty decent for a kid, there's tuition reimbursement (or WAS, but I won't go into that), and you will learn exactly what you're made of as far as work ethic right away. Many don't make it --"it's just too hard," However, if you have the grit in you, it offers a springboard into several paths at UPS once you get your degree, and not just as a driver.
While these guys gripe and talk a good game of how much they hate their jobs, they're really a lot of fun and pretty cool for the most part from my perspective over the last 10 years. If you do chose that path (after school and additional self-development on how the world works, etc), you certainly will for the most part, be part of a really good group of people.
Best of luck to you and stop being so serious at 15--go play around on Facebook or something and get off this trainwreck--ENJOY your youth!!!!!