To decrease, you crochet more than one stitch together. The one I use the most is crocheting 2 single crochet stitches together.
1. Insert hook into next stitch and draw up a loop:
2. Insert hook into next stitch and draw up a loop:
3. Yarn over and draw the yarn through all 3 loops on the hook:
This is what it looks like after completing 3 more sc2tog:
Crochetably 