As this used to be the most crime-ridden area of Amsterdam (and the Netherlands as a whole), it is important to always be cautious and stay within the well-populated areas. Do not walk around alone in the dark and avoid groups of youth standing around. Generally speaking, the area around Bijlmer ArenA station and along the Bijlmerdreef is quite safe, while the further off you get from there, the unsafer it gets. Currently the area around Holendrecht is probably the worst. It must be said though that the last 20 years crime has lessened considerably and the area can be visited safely during daylight.
Non-residents who never actually go there will tell horror-stories about Bijlmer, but not only are they generally out-of-date, they also need some perspective: even though populist Dutch politicians claim otherwise, the Netherlands is one of the safest countries on earth, and Amsterdam one of the safest large cities. If you live in a city, chances are that it has neighbourhoods much less safe than Bijlmer. Even within Amsterdam there are much worse neighbourhoods, such as the western suburbs of Slotermeer/Slotervaart.
Objectively Bijlmer may even be safer for tourists than the center of town - pickpocketing is by far the commonest crime tourists suffer in Amsterdam and it is endemic in the center and on the trains to and from the airport. As there are almost no tourists in Bijlmer, there are hardly any pick-pockets either!