It completely depends on the style you're brewing. I like first wort hopping for the first hops addition to an all grain brew (a suggestion from a more experienced brewer).
In general, early hops additions will add bitterness, and late hops additions add flavor and aroma. I usually use at least two hops additions, if not three or more. Generally, the first is at the beginning of the boil, the second at 15-20 minutes remaining, and the third at flameout. The amounts vary by the type of hops I'm using and the effect I'm trying to get.