I recently did as Bats explained with an all grain wheat recipe.
Just racked onto about 1kg of frozen raspberries (which I 'potato' mashed with a 2 cups of boiling water. This was after 10 days of fermentation. Once racked onto raspberries, I kept at fermenting temps for 4 more days, then dropped the temp to about 3c for a further 4 days, before kegging.
Was really happy with the result. A nice pink tinge to the beer as well as an awesome raspberry aroma and taste on the palate. A winner with SWMBO aswell, which always helps.
Don't cook fruit then add. All you get is a stewed fruit character. Which 99% of the time is not what your after. I'd suggest having a read of the wiki articles on here about fruit and their use in beers for the better way of doing it