It depends on your situation. If you are simply porting there, then you can easily walk into town. Just leave the cruise terminal and walk about 15 minutes to the right, go through the train terminal and you will enter downtown Amsterdam. There is a tourist information center there where you can pick up information about Amsterdam's sights. and a map. If you are embarking or disembarking there, then you will need to grab a taxi into / from town with your luggage. Amsterdam is a great port and you are sure to love it. Here is a link to tons of information about Amsterdam
Tom/shanghai, thank you for the great links. We will be embarking in Amsterdam this summer, along with our 13-year-old son. So, although the red light district will likely not be on our list, it will still be difficult to narrow down what to see/do. The Anne Frank house is a definite, as is a canal boat tour, and lots of walking around. We'll have two days pre-cruise and one day post-cruise to enjoy...and I am having as much fun researching Amsterdam as I am our ports of call of the cruise.
I booked some tours thru Viator.com for our stay over in Amsterdam after the Ship gets back in May. They have lots to pick from and you print your ticket right online. You can read reviews and I felt the price was great.