For the definition of unknotting Number, you can assess http://www.popmath.org.uk/exhib/pagesexhib/unknum.html

My question is:

For given a knot K, let n be the crossing number of K, is their any estimate of the unknotting number of K? Of course, the unknotting number is smaller than n-1, but does there exist any nontrivial estimate?

I guess the unknotting number may be smaller than $[\frac{n}{2}]$. Based on the fact that the unknotting number of the torus knot is no bigger than $[\frac{n}{2}]$. I think the torus knot might "tight" in the most "fierce" way.