There a good section on Fixing Mistakes
under the Free Videos tab at the top of the page, Tips. Amy covers many of the common mistakes, the cause and the fix. While you're practicing, it's good to try recognizing mistakes and fixing them, either by taking out a row or working over to the errant stitch.
Also, for slipping the first or the last stitch. Do one or the other, not both. If you slip the first stitch of each row, you'll make a nice chain edge up both edges anyway.