The essentials
Molar concentration is amount of solute in moles divided by final solution volume in litres: c = n/V.
Use the final solution volume, not simply the volume of solvent added. This is a calculation lesson, not an instruction to handle chemicals.
Worked example
Dissolving 0.5 mol of solute to make 2 L of solution gives:
c = 0.5 ÷ 2 = 0.25 mol/L.