It's really hard to make a silk purse out of a sow's ear. It's always best to start with a very high quality image. You can always sample down but up-sampling usually results in more distortion than what you started with in the first place.
Image > Image Size. Add height, width and pixels per inch. More pixels = more to work with.
Apply Adjustment Layers.
Experiment with Filters > Sharpen edges, Unsharp Mask, etc...
Good luck!
Nancy O.