You also need the volume or the density, depending on the problem. You can't find concentration without some idea of the volume.
Molar mass (molecular weight) (grams/mole)
Number of moles (moles)
Density (grams/mL)
If you have the volume (for example, it's a solution), you can divide by that
If you have the density, you can calculate the volume occupied by the material:
Since you now know the mass (g) and the volume (mL), you simply divide them to get g/mL.