Just be attentive of your surroundings and you will be fine. If a place looks sketchy, they don't go! Many great places in Detroit to visit, but plenty of bad areas too. Much like any city.
If you are in the downtown area, you should be ok as long as you keep with another person. That area is a bit safer for tourists. Wandering into the neighborhoods, and particularly on the west side, is poor idea, steer clear.