There are 2 type of sc decrease methods in crochet. The sc2tog, and the invisible sc decrease. I highly recommend using the invisible decrease in amigurumi, as you will achieve a better quality and your piece will look better.
sc2tog:
1: Insert your hook under both loops, and pull up a loop. (you have 2 loops on your hook)
2: Insert your hook in the next stitch and pull up a loop again.
This point you have 3 loops on your hook.
3: Yarn over and pull through all 3 loops.
Invisible Decrease:
1: Insert your hook in front loop only (FLO) (2 loops on hook)
2: Straight after that insert your hook in the next stitch, but again only in the front loop. (3 loops on hook)
3: Yarn over and pull through all 3 loops.
Using this method, your decrease will more look like a sc, and your finished product will be smoother.