How does one go about figurin his/her bike's frame size? I'm 5'9'' (approx. 175 cm) And why does that factor matter? (I'm new with this biking thing... and really want to know more!)

Well sizing matter because first of all you need to make sure the top tube of the bike is low enough that you can stand over the bike without hitting your crotch on it. If it does it will be extremely difficult to mount/unmount from the bike and an emergency jump off the saddle could be extremely painfull regardless of being male or female. Slaming into a metal tube is painfull no matter what. That is called standover height. The distance from the saddle to the handle bar is also the most important measurement. This is generally known as top tube length. Too long and the controls will be dificult to reach and your back will hurt after a while as well as your hands and butt since your pressure points will not be properly distributed. Too short of a bike can also hurt your back since it might be force to bend too much. Steering can be clumsy, your elbows can bump into your knee's when pedaling and the front wheels can even bump into your foot when turned when the pedal stroke is all the way forward. This will very likely cause you to crash. You will also feel a tendency to go over the handlebars during a hard brake or downhill. Proper bike fit is the first and most important factor to consider when buying a bike. Going to a bike shop to get a good fitting is the ideal way to go. From experience, most people in your height range tend to require a medium sized (typically about 17" to 18" size). If you want something technical to read about bike sizing check this out: http://www.coloradocyclist.com/bikefit/ It talks mainly about road bikes but also explains mountain bikes sizing.

Oh - so awesome - THANK you! i really had no clue any of that could be affected. And yes, going to a bike shop to be fitted sounds like a great idea.