One of the simplest and most effective ways to achieve flattering photos with a speedlight is to bounce the flash. You usually never want to directly illuminate the subject as the light is harsh, unless of course you prefer "The Deer in Headlights Look".
What you will need:
1 White Business Card (the more reflective the better)
1 Rubber Band (optional)
Generally on Nikon speed lights there is a space between the wide angle diffuser and the strobe and the top of the flash gun. Simple solution: use a business card.
Step 1: Point the flash head upward
Step 2: Fold over the the to bottom corners of the card in the portrait aspect several times. Repeat this back and forth until you are able to remove the corners.
Step 3: You should now be able to place the card in the slot between the wide angle diffuser and the strobe itself. If your speedlight does not have this you can use a rubber band instead.