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.

I had a problem with my rotary cutter once and found that I had two blades on instead of one

Rush88888 covered most of it. You need to be sure your ruler remains still, but it is also possible to waver from the edge of the ruler if you are nit careful. Try to hold the cutter at a consistent angle. I use a June Taylor Shape Cut (1/2") for almost all of my cutting because I have to sit while I cut and it make it easier for me to keep my cuts true. as you go along, please remember to sharpen or change blades regularly. Dull blades chew the fabric, rather than cutting it and are much harder to use. As much as I hate ironing, having your fabric freshly pressed does matter in getting good cuts.

This is the best tip a friend told me about to keep my ruler from getting out of line.
Buy a bathtub suction-cup handle like this. By putting this on my ruler, I can get a better "grip" and it stays in place REAL WELL when I make those cuts (large or small)

I too used to have the same problem as you. Until a friend told me to be sure and keep one finger on the fabric while holding the ruler. Made all the difference.

I had a bit of a problem with ruler slippage but I bought Grip Strips from Guidelines4Quilting.com. They let the ruler slip over the fabric when lining up but when you put pressure on the ruler to cut they grip the fabric and no more slippage.
Not bad for $10.00 you get 6 strips - 2 on your 12" ruler & 4 for your 24" ruler

I straighten my fabric like in the video. Thank You Lois for posting the video because I could'nt explain it in writing. I was taught to do thid by a quilting teacher years ago.

Thank you everyone for these great tips! I am a lefty, and was wondering if there was an "issue" about placement of that blade. I keep bumping the knob on my ruler. Going right now to change it!
And even as a noob, I realize how much better it would be with bigger equipment. I'm currently using an 18" fiskars self-healing mat, and a 6x12 non-slip ruler. They're ok and do the job just fine, but it would be better to have the wonderfully large cutting spaces that many of you have.
Oh, and there's a lot to be said for a table that's the right height...holy cow a 6' folding table is too low! Must put some 2x4s under the legs!

Once I get my ruler exactly where I want it I put a weight on the end furthest from me, just to help in keeping it in place as I tend to unintentionally loosen up on the ruler while cutting. But I also walk down the ruler as I cut as someone else described. Cut some, move my hand up the ruler, cut more, keep going till I'm done.

My "weight" is really just the head of a smallish sledge hammer that I made a fabric cover for so you can't tell what it is(and contrary to his "I'm gonna find a handle for it soon" speech), DH hasn't even noticed it's not in the garage anymore. I use it all the time for so many things while sewing and cutting. I also "borrowed" a 2" socket from the garage(he keeps all his real tools at work anyways) and turned it into a weight/pin cushion.