Are they brown-ish marks or are they more purple/red? If they're more brown in color, hydroquinone would be your best choice, as well as an AHA, retinoid, kojic acid, etc. Do a bit of research on those products and you'll find what works for you.

Alternatively, you can ask your dermatologist. By doing so you'll be able to get prescription grade versions of the things I mentioned above, allowing for greater strength.