As a testament to Apple's dependability and quality, check ebay. Go ahead. There are PowerBooks from NINETEEN-NINETY-SIX that are selling for over $150!! How many other laptops from 1996 are selling for close to that?
As well, I recently used an iBook from 2000 as a gaming notebook. Being ten-years-old it still boots and runs like new! I was flying on some great old 3D games. It had no issues what-so-ever.
Macs last. It's just a fact.
*as there is a dead 3-year-old Compaq Presario F500 sitting on my desk*
You will save money if you buy a Mac, believe it or not! ZERO antivirus software needed. It comes with hundreds of dollars worth of "lifestyle" software, such as iPhoto for editing, managing and sharing photos, iMovie, for editing videos, iDVD for creating full-featured high-quality DVD's, iWeb for creating and managing a webpage, and Garageband, said to be one of the best music-creation devices. Garageband has, in fact, been used to cut entire albums you hear now.
As well, if you buy a $500 laptop now, you'll have to buy ANOTHER in three years when it's on its last leg or outright dies. That's $1,000 in four years for what? For what reason should you pay $500 for something that will need to be replaced for manufacturing faults? Buy a MacBook for $1,000 upfront, and it will still feel fresh four years down the road, and run for well over a decade.
There are countless web pundits that have posted figures proving that it is, in fact, far more affordable to go with Apple than Microsoft and their OEM's.
However, at fifteen, you should probably just save your money, as another poster said, and use what you have available now. For what reason do you require a full-fledged notebook? Why not go for a netbook or an iPad, assuming you have another computer with which to sync it?
At fifteen, I was a huge computer noob, though I had used computers since I was five. I didn't get serious until I was seventeen, and oddly enough, bought a mac

Just wait. You probably won't be serious about computers for a number of years, and by then, you'll be glad you waited for the better hardware.