Deciding between frost and fire is up to you. There are plenty of threads that will give you all the information you need to make that choice. Once you decide, there are threads that will tell you how to optimize your performance.
I absolutely LOVE my fire mage, and wouldn't switch for the world...but then, I know a lot of people who say the same about frost. I started frost, actually, then switched because I'm a pyro, basically, and I love teh burnage :P