well am shooting at night .. so i guess my flash will be my key light ..
and the street lights will be my ambient.. or if i shoot in the day i still
would prefer my flash light to be my key light .


meter ambient light, select an f/stop that can give you enough flash
exposure and enough ambient light.

remember:

flash is controlled by the f/stop and output power
ambient is controlled by the f/stop and shutter speed.

your flash sync speed (for focal plane shutters) is the maximum speed that
will allow the flash to fire while the shutter is open fully from one side
to the other, you can usually shoot at slower speeds and many cameras have
choice of first or second curtain flash (so the streaks can look like they
are following the subject and not streaking through them)

so you could select a large f/stop or low output from your flash and then
look at your meter and select a shutter speed that will allow the ambient to
build up.

if your flash is the key light then you want your background to be one stop
under, maybe 1.5 stops

if your flash is fill then select an f/stop or output power that will be one
stop under the ambient exposure on the subject.