Amsterdam is generally a safe city, particularly when compared to other world cities.
You should, of course, keep a close eye on your personal belongings, as you would in any city – there are pickpockets around, particularly in the city centre, in busy areas, at tourist sites and on public transport.
If you buy or rent a bike you must always chain it up very securely, as bike theft is quite common. And it is sensible to avoid using ATMs at night in quiet areas.
But violent crime is rare, and Amsterdam usually rates among the safest world cities. As long as you take sensible precautions, as you would in any city, then the chances are high that you will have a trouble-free experience.