A single crochet (SC) is a two step stitch by inserting the hook into the desired stitch (usually) below, yarning over (hook wrapped with the yarn) & drawing up a new loop, yarning over and drawing it through the loop just pulled up & the one on the hook. A Double Crochet stitch is made in a three step method & twice as tall. Yarn over the hook before inserting in desired stitch, yarn over & draw up a new loop (3 on hook now), yarn over draw through first two loops, yarn over and draw through remaining two loops-DC completed. There is an additional stitch between these two called a Half Double (HDC.) The height is half way between as well. Let me know if you need additional help.