Molarity and molality are both used to express the concentration of a solution. The amount of solute is both expressed in moles.

Their major difference is for Molarity, it is expressed as the moles per Liter/s of solution while for Molality its moles per Kilogram/s of solvent. 

Molarity (M) = moles of solute / Liter(s) of solution

Molality (m) = moles of solute / Kilogram(s) of solvent

Take not of the use of symbols (capital letter M for molarity and small letter m for molality)

To avoid confusion, always remember that:

Volume of solution = volume of solute + Volume of solvent

OR

Kilogram of Solution = Kilogram of solute + Kilogram of solvent

Tip: you can use the density of the solution in working with this kind of problem.