Let's dissect this a bit for you, seeing as it's your first computer.
A Mac is more expensive. A Mac doesn't have as much of a range of software available to it (this can be a blessing however; there aren't many viruses for Macs for this reason... Apple is very tight with their code. So you're trading off a more expansive library of available software for some extra safety). A Mac can't be upgraded (as easily as a PC). However a Mac does boast a nice UI (user interface) and what most consider to be easy computing. Apple stores, from what I hear, offer excellent customer service and are very helpful, but again with Apple you'll have to shell out a lot more for the privilege.
A PC can be a lot cheaper which is definitely a plus. If you do you research and buy separate components tailored specially for what you'll be using it for you can save a fortune as opposed to buying on off the shelf, or even buying a Mac. While it's true that PC's are more vulnerable to viruses than Macs, if you get some decent antivirus software such as Nod32 and use some common sense this won't be an issue. I haven't gotten a virus an a time I can remember on my PC. With a PC you can also upgrade it as you need to... say your graphics card gets out of date or carks it, it's extremely simple to just get a new one and install it. Not so with a Mac.
That's just some stuff off the top of my head. I'm not pro or anti PC or Mac, I'm just stating facts. Personally I use a PC loaded with Windows 7 at home, however at work as a multimedia designer I use a good mix of both PCs and Mac. When it comes down to it it's whatever gets the job done.
In short, as I assume you're fairly young and don't have a massive budget, I'd recommend a PC. If you do some research and buy the separate components in order to build your own one you'll save a bunch and it's also a good learning experience.
THAC XIV entry here:
(Never) Meant To Be