Help: Tightening Headset

Sorry if this is a really obvious question, but I noticed that when I was using my front brake there was a tiny 'clunk' sound. Stopping at the lights it was obvious that the nut on the top of the headset was really loose.

I have an adjustable spanner that I can use to tighten it up but I wondered how tight should it be? Just tight enough or very tight?

Look on the park tools site for visuals and guidance

http://www.parktool.com/blog/repair-help/threaded-headset-service