Firstly I'd suggest reading the Amersfoort meet threads. Although not about Amsterdam, I'd guess that some of your Holland-specific questions will have already been answered there.

You will find that just about every Dutch person speaks excellent English, so you don't have to worry about major language problems. Money-wise, you'll find that plastic will work in a lot of cases. If your debit card has 'Cirrus', 'Maestro' and 'Pulse' then you will have a hard time finding an ATM that won't give you cash. It's got to the point where I don't bother getting any currency or travellers checks anymore when travelling to a first-world country.
As far as Amsterdam is concerned, I'd class it as a common-sense city, ie there's no need to be paranoid, but do take common-sense precautions. Avoid hanging expensive cameras around you neck or keeping your wallet bulging in your back pocket. That sort of thing. Don't buy anything off the street - the coffee shops care about their reputation unlike Mr Random on the street corner. Amsterdam relies on their tourism far too much to tolerate too much anti-social behaviour, so even the red-light district is relatively safe.