First step is to make a good hefewiezen. Fortunately, this is a pretty easy style, and one you can do very nicely with extracts. You should base it on what wheat-malt extract you can find (should be in the range of 40 - 60% wheat). Adjust amount to get you in the 1.050 range (+/-, as you like). You don't need much hops - 1oz of Hallertauer at 60 min will be fine. Brew/ferment as usual. Then, rack onto raspberries (frozen, thawed is a good way to go). Amount? Depends on how subtle vs. dominant you want the raspberry. Remember, raspberries are very tart, and you will not be able to balance that with sweet in a beverage you'll be bottle conditioning. How about 2 lbs?