You've received great advice from everyone but I would also suggest you do a bit of practicing on
cheap cuts of fabric (such as muslin) and not something you've planned for your first project. And do yourself a favor and watch these two videos: "How to Square your quilting fabric" (before cutting)
http://www.youtube.com/watch?v=EcpzwJMVTbc and "How to Cut Fabric with a Rotary Cutter".
http://www.youtube.com/watch?v=ybQLai6Mv58
Hope this helps.