Here is what you need to know:
Apple: Makes excellent laptops. Not as good a value as a PC. But a good high quality product. Avoid anything that is version 1.0 by Apple. Please see the original I-Pod non-replaceable battery fiasco. Also see the same problem with the new I Phone. Apple has a cult like following so don't ever attempt to debate an Applehead just let them enjoy their 5% market share. Apple is coming out with a new line of Laptops soon so you might be able to pick one at a good price now.
Windows: You have to be careful when buying a Windows PC. Please avoid Dell. They have had more recalls than any other laptop manufacturer. Also their service and support is terrible. The best laptop manufacturers are Acer, Lenovo (formerly IBM) and Toshiba. DO NOT buy an entry level laptop. At this point I would suggest you get XP-Pro. Vista is still getting the kinks out and Microsoft will support XP through the end of your school experience. You have to decide how you are going to use the laptop before you decide what model to purchase. If you are looking for a desktop replacement, you should get a larger screen with a wide screen. If you want to lug it around, you should look for battery life and weight. Unless you are gaming, XP-Pro with 512 MB of RAM will be fine for browsing and doing basic office apps.
Linux: This is best avoided unless you are a propeller head (like me). You need more than general knowledge to play with Linux. It is really secure and an excellent OS, but not for beginners. Let's all say recompile the Kernel.
If you have opportunity to buy insurance that includes accidental droppage, spillage, and other user induced damages, that is recommended.
Finally, the most important advice I can give you was given to me when I was learning to program in d-Base II and it has stuck with me for almost twenty years:
THERE ARE TWO KINDS OF COMPUTER USERS, THOSE THAT HAVE EXPERIENCED HARD DISK FAILURE AND THOSE THAT ARE GOING TO.
What that little fortune cookie wisdom means is, you should make sure you have
regular method of backing up your data files. I would recommend a 2.5" external hard drive with a USB connector. You should also e-mail important documents to yourself so you can get them from any PC. My computer crashed is the 80's equivalent of the my dog ate my homework and is not likely to be accepted by many professors.
Finally, whatever OS you choose, you must make sure you keep it up to date. OS X has security flaws and needs updating the same as Windows, though not as often and the same goes for Linux. Hackers are looking to take over your computer to use it for nefarious purposes and that means they don't want you to know they have succeeded.
This little public service announcement comes from someone who (in addition to being a pharmacist) has run a PC business for the last 15 years. That means I make money from college students who don't take care of their computers, don't backup their computers and generally take them for granted.