Welcome to the world of quilting! You do not need a rotary cutter at all to quilt. They were only invented maybe 40 years ago & long before that quilts were constructed using scissors.
Rotary cutters offer a more precise cut than scissors. If you are going to use a rotary cutter, you also need a self-healing cutting mat (I love Olfa brand available at JoAnn -- mine is 18" x 24", you don't want to go much smaller than that; you can buy bigger ones if you'd like) and a ruler. A 6" x 24" ruler is probably ideal but I've been using by 3.5" x 24" ruler that I bought when I started quilting for many years now & it's usually sufficient. I like the Omnigrip rulers, but there are lots of great rulers out there & just as many opinions about which is best.
For your rotary cutter, there are 4 standard sizes: 18mm, 28mm, 45mm & 60mm. 45mm is the most common for quilting. The size of rotary cutter you have will dictate what size blades you buy. I'm not aware of a rotary cutter that lets you switch between different sized blades. I only had a 28mm cutter for years. It worked fine for everything except making cuts using a specialty ruler called a ShapeCut. 28mm cutters will only cut through 4 layers of fabric at a time. 45mm cutters can do 6-8 layers & 60mm cutters can do even more. 18mm blades will only cut through 1-2 layers at a time, but their small size allows them to better cut along curves and intricate designs.
You do need to buy replacement blades in whatever size your rotary cutter is (if you don't know, check the blade itself). A good rule of thumb is to change your blade each time you start a new quilting project. I would recommend buying a 5-pack to start with so you have extras if your blade starts to dull & not cut through all your fabric. Dull rotary blades, like dull knives, are dangerous. They can slip, jump on/over your ruler and slice your hand. Not fun.
Wal-Mart has the best deals on rotary cutters & blades.