There are an infinite number of coffee shops in Amsterdam, and it's very odd because they're still operated and maintained like they were when they opened, when the sale of marijuana was illegal, so they feel very clandestine at times. Go down the stairs into the basement and there you'll find a counter with a menu displaying everything for sale and the corresponding prices. You will see the warning light that advised the salesperson in the basement if the police were coming so that he/she could hide everything in time, you will also see the microphone used for communication with the upper floors. Here you can buy different strains of marijuana in addition to pre-rolled joints (€12 for 4 joints). If you don't want to buy any of this, you can always have a delicious mix of espresso and hot chocolate at Café Bulldog (€2.60)..
If you consider yourself to already be a marijuana expert, then it's better to go to more specialized shops like the Blue Bird (Sint Antoniesbreestraat 71, very close to Nieuwmarkt), or the Dampkring (Handboogstraat 29, close to Spui Square).