watash1965,
Your questions about aperture and focal length are going to depend on a couple of things:
1) Do you want to just focus on the subject, or do you want to show him or her in the context of the environment? If you just want to show the subject, you can afford to stand back and zoom in a little, i.e. a longer focal length. If you want environmental context, you might want a shorter length with a wider field of view.
Read up on famous street photographers and see what they liked to use and why.
2) Your decision might also depend on how comfortable you are in getting close to people. If you are shy, or leery about getting close, or if the person you are photographing in uneasy about you getting too close to them, you might want to stand further back and use a longer lens.
As far as keeping your camera safe, you might consider a wrist band. I also have a shoulder holster with a stomach band. That way, even if someone were to snatch it off my shoulder, it's still attached to my body.
Steve Thomas
