Brenda,
A teacher told me to fold the selvages together. Hold the fabric up and let gravity help you. Now shift the selvages left or right until there is no twist in the fold area. When you have it straight lay it down on the cutting table. Now, line up a line on the ruler with the fold and trim the edge of your fabric. The rest of your cuts should be just fine, as long as you take time to line up the ruler with the fold.
Once you move the fabric you may have to straghten up the edge again. Take your time. The old saying about haste making waste is so true in quilting! :?