One of my cousins, a high school sophomore, wrote to me and my brother asking for advice on purchasing a first computer for high school and college. She's comfortable around computers -- she spent the summer she was 12 repairing Linux-based laptops with me in Chicago -- but that's not her primary focus, so she's probably looking for a fairly generic machine for academics and random projects (which may or may not be about computing).
First things first: my brother Jason recommended "something portable with good battery life" for group projects. I agreed.
Remember, batteries are replaceable, and you are likely to be in for new ones
every two years no matter what. If you plan on keeping your laptop for 4, that's just
one battery replacement, so that's not as bad as it sounds.
I'll second the portability recommendation. As a Linux geek, I've found Thinkpads (I have the x200) to be well-built machines; they usually ship with Windows if that's your cup of tea. But if you like Mac, the Air looks lovely. I haven't been as impressed with Dell, Toshiba, Acer, and Gateway; their consumer-grade stuff seems to break quickly under normal use.
I'll also note that it's unlikely that you'll keep the same laptop through 7 years of high school and college combined -- things wear out and break, and hardware expectations get more powerful, so think of this machine as a 3-4 year purchase, something you're likely to replace either as you start college or after your freshman year (once you have a better idea of what you'll be doing for the rest of your college studies). Some college programs require you to purchase a specific laptop, and most will give you significant discounts on computer purchases, so you're really thinking "what's going to last me 'till college?" With that in mind, a used laptop in good condition is pretty appealing, as might be a netbook plus external monitor.
The next thing we both commented on were accessories. Jason recommended getting an external hard drive, monitor, keyboard, and mouse, something he figured out his junior year at Stanford. I started getting clear signs of RSI well before my junior year at Olin, so I concur; ergonomics are worth it.
You really do want this stuff; at the very least, stick the laptop on a stack of books so the screen is eye-height, and get an external keyboard and plop it on your lap so your wrists are straight and your arms make a (rough) "L" with your shoulders down and relaxed. If your laptop screen is small (below 12", which likely means you have a netbook) spring for a nice external and you can take it to college with you (I did, and the... hrm, maybe $700 I spent on a 24" monitor has well exceeded its return on investment -- it's been with me for nearly 7 years now).
A bit of backstory: I got my first computer when I was 15, after 5 years
of working and saving towards exactly that goal. That cheap little
Compaq -- that, and a bunch of older buddies with a stack of Debian
install floppies -- changed my life. So I know how monumental this
decision can feel, and how unfamiliar the territory can seem (I didn't
know how to learn about computers, but I sure did want one), and
how impossibly expensive everything can be when you're a teenager who's
not allowed to think about having steady external income sources yet. I literally saved until I could buy the cheapest desktop at the local computer store, then bought it. That was all the strategy I could think of, and at that age, I didn't know anyone else who was more well-versed in computer purchasing.
Of course, my 15-year-old self is a decade back, and things have changed considerably since then. If I were a 15-year-old now and budget was a primary concern, I'd look for a used Thinkpad in good condition. If I were thinking longer-term, I'd put most of my money into a good large monitor that could be reused in future setups, and get a reasonably oomphy netbook or smallish used laptop (perhaps the aforementioned Thinkpad), and install whichever operating system I preferred with open source office, art, etc. software. Of course, my product-design brother recommended the MacBook Air, which... may fit into a very lucky 15-year-old's budget, but certainly wouldn't have been within mine.
Anyway. That's what we told her -- anything else you'd suggest or note or recommend?