Resolution is not the primary issue, but in general scan at 300 or 600 dpi.
The real issue here is properly setting the [typically] 3 major exposure
controls, brightness, contrast and gamma (these have different names in
different software programs, but in most cases there is one for dark
areas, one for light areas and one that controls how the dark-to-light
transition is handled).
As to resolution, if the photo is small (less than 3.5" x 5", certainly)
OR if you want to print a large output, use 600 dpi, in most other cases
300 dpi is fine.
But, again, exposure settings are far more important.
