Rotary cutter sets are generally expensive, but you can get a good deal if you wait for a sale at Joann's, or buy when not on sale and you have a 50% off coupon.
I prefer the Olfa blades to the Fiskar blades, but you can buy a cheaper Fiskar set and buy Olfa replacement blades to cut with. My set uses an 18" x 24" cutting mat, a 6" x 24" cutting ruler, and a 45 mm rotary cutter.
The rotary cutter works simply by allowing you to layout your fabric on the mat, line up the cutting ruler where you want to cut, then run the rotary cutter along the edge of the ruler. You will get a perfectly straight cut every time. If you use templates, you just mark the corners of your template on the fabric, then line up the ruler from point-to-point and cut. If you don't use templates, both the cutting ruler and the mat are marked with grids that allow you to measure your cuts and line them up parallel, perpendicular, or at specified angles on the fly.