Newbie, the obligatory smoke you just have to try if you're in the Dam are Amnesia Haze, and the classic White Widow. I mostly smoke hash while I'm out there, though, nice Moroccan available at a few places. There's a lot of shit coffeeshops, some of the better ones include the Rokerij, Boerenjongens (out of the way, but if you're there, make sure to try the White Choco), and the Dampkring. Best places to buy aren't necessarily the best places to smoke, but you can pretty much smoke anywhere. You should check out the Van Gogh museum, the Nieuwe Kerk, and if the weather's nice, the Vondelpark.