Hi,
You and your friends are gonna have a BLAST! Despite Leafminer's nightmare it's a safe city to explore and visit. I lived in the Amsterdam Centrum area from '87 til '91 and I was never robbed or the victim of violent crime but just like any large city you have to keep your street smarts in check.
First thing... don't bother learning Dutch! In addition to having sounds virtually impossible for an American tongue to learn in a short time Dutch spelling and grammar have entirely different pronunciation rules. It's great to learn a few words and phrases to show respect but Amsterdam is one of the easiest destinations for English speakers to visit in Europe. The vast majority of people speak perfect English albeit more of a British style than American. Many of the older generations don't speak great English but they know enough to do basic business. If you don't have your hotel plans nailed down do it sooner than later if you want to have any selection or save any money. Maybe with the current economic slump there will be plenty to choose from but usually it's not the case and they keep pretty booked up long in advance.
If you're staying in the Centrum area - the center of the city which fans out in canal rings from the central train station - you can walk just about anywhere you'd like to go but public transportation is easy and very convenient with trams that you can hop on and be anywhere you'd like to go for a very reasonable fare. You can buy a strip of tram tickets called a Strippenkaart when you step out of the train station upon arrival at the GVB office which is in a small building right in front of the station. Pick up a transit map while you're there...
There are fewer coffee shops than there used to be due to a trend of cracking down on those who don't follow the rules. The ones that get in trouble don't get to have their license to sell mj renewed and they are shuttered.
Not to worry though... there are still hundreds to choose from and you'll have to sample around to find what style of coffee shop suits your style. I have a few faves... The Greenhouse has many locations and always top shelf goods. My favorite location is the Greenhouse out on Tolstraat if you'd like to take a tram ride out of the center and for that matter there's a great one called Trinity that's more frequented by locals just outside of the center. A couple more worth going to are De Rokerij (pronounced De Rokeray cuz ij makes and 'ay' sound in Dutch), The Grey Area ran by a couple of American expats which is very small and crowded but usually has great bud, and I'd have to throw in Dampkring as well for their quality.
Now for a recent curve ball I thought would NEVER happen... last year they banned indoor tobacco smoking in Amsterdam! Wow... as much as the Europeans love their tobacco I never thought they'd enact such laws. Never say never... I'm not a tobacco smoker and when I go to Europe it's one of the most unpleasant parts of going to restaurants and bars because a larger segment of the population seems to smoke and all businesses were always thick with tobacco smoke. I have heard that some of the coffee shops are turning a blind eye to tobacco smoke... if they don't "see" you smoke tobacco they don't do anything but how long will that last? Having said that, the coffee shops are really down in business due to this new law. As many of you know those wacky Europeans like their weed with tobacco and now it's a no no and people are buying weed and leaving to smoke it at home or their hotel where they can combine it into spliffs with tobacco. I can't stand tobacco in my bud... to each their own though... I remember once sitting in a coffee shop with an American pal who came over to visit when I lived there. He was so excited... he had just bought a couple grams of the awesome sativa Kali Mist and a gram of temple ball hash. As he put a lighter's flame to the ball to soften the hash and then combine it all together for a humongous joint we noticed there was a table of Brits watching our every move. They couldn't believe that we were not only rolling it all into one big joint but we were going to smoke this lovely combination straight without any tobacco... As we proceeded to smoke our 20 minute joint they watched us hit it and cough and I really think they were waiting for us to pass out. At one point my buddy offered them a hit and one of them said: "Oh GOD... Crikey no!"
Any-hoo... in addition to all the bud you could ever desire Amsterdam is such a fun city to visit, especially in the summer. The architecture and canals have so much history. Ah... lazying in a street cafe table with a nice Dutch or Belgian beer and a fatty... And there's a lot more to do than smoke bud. Somebody mentioned the Van Gogh Museum and it's the place if you're a fan of this prolific impressionist like I am. If you still want more Van Gogh after that, there's a great little museum called the Kroller Muller Museum in a national park of all places that is about an hour's train ride from central station. It's a great day trip and would be very nice to go to in the summer. There are white bicycles all over this park that anyone can use too. The beach isn't far and can be a fun outing or even a trip out of the city to see some of Holland is a good time. Just about every town of any size has sights as well as coffeeshops to explore.
Have a great time and stick to bud and hash if you want to stay out of trouble. Only buy your weed in the shops as the dudes who are peddling hard drugs or anything for that matter are very shady and to be ignored. As long as you mind your p's and q's and keep your wits about you you'll have a great time.
Peace!