The bike size and fit are very important. The more you ride, the more important fit is. And likely, the better your fit is, the more you will ride.
There are some pretty good examples of bike fit calculators. Here is one I remember working pretty well and having a lot of information:
The less used to riding you are, the harder the decision of frame size is. Since it's new to you, expect to make adjustments as you go. You will be able to use a more aggressive riding position as your flexibility and certain types of strength improve. (that's better if you'd prefer to go faster with the same exertion)
If I remember right, some folks will say that a cyclocross racing frame should be 1-2cm smaller than what you would use for a road frame. If you're going to use the bike for transportation, ignore that advice. (That advice is only because being positioned further forward might make it easier to "steer" around corners in cyclocross racing. The difference being that in a cyclocross race, you'd be navigating sharp corners on grass and mud as fast as possible.... much different than the type of turning/riding you do when commuting)
Generally, bike sizing is done by getting the correct top tube length. If you have the top tube length correct, and you're fairly flexible, the seat tube length will work out ok. (the heat tube height can also be of importance, especially now as frame makers are using a wide variety of HT lengths). The thing is, knowing what top tube length you want can be hard without a lot of riding experience and position experiments, or without the help of a good bike fitter. A bike shop salesman is often NOT going to give you good fit advice (Definitely don't just take their word for it if they tell you something like to just stand over the bike and if there's 1-3 inches clearance, you're good). There are very good bike fitters scattered around the US. Here's an example: http://cronometro.com/biodynamic-fit/. Bike fitters can be expensive and it can be hard to tell what you're getting into before the actual fit.. The examples of people you will hear about from word of mouth as being really good are generally the expensive ones, and you don't need that kind of an in-depth fit yet..
I hope this helps some... feels like I I've just been rambling.