Whatever you get (Mac, Windows, Linux), make sure you install the 64-bit version of the OS. There’s no reason to be stuck with 32-bit limitations with any of the modern operating systems on the market today. Consider what software and media you plan to interact with on this computer. Do any of your classes require a specific OS? Might there be a combination of classes where you’d need all three (consider Mac’s Bootcamp)?
Besides that, do yourself a favor and go physically touch different machines from different vendors. Pick them up. Type on them. Try the mouse. View the screen from different angles. Don’t purchase based on emotion alone.
And feel free to upgrade the user-serviceable parts of your machine after you buy it. Install your own RAM. Upgrade your own hard drive. Do the pricing research and you’ll often (but not always) find that you can do some of the simple stuff yourself for much less than customizing/upgrading at time of purchase.
Don’t assume you have to buy big name, expensive software packages from known vendors. You don’t have to get Microsoft Office (expensive, Windows only), for example. Consider Open Office (free, any platform) or iWork (cheap, Mac only). Same goes for GIMP vs. Adobe Photoshop.
Budget ongoing payments for anti-virus software if you go with a Windows-based machine (absolutely necessary). Do not buy any anti-virus for the Mac (there are simply zero in-the-wild virus exploitations for Mac). I’m not qualified to discuss Linux anti-virus requirements.