In your case, sodium hydroxide is the solute and water is the solvent.
Since you know how many grams of sodium hydroxide you must dissolve, you can use the compound's molar mass to see how many moles you'd have
Now you need to convert the volume of the solution to kilograms. This is done by using water's density, which you can assume to be equal to 1 g/mL.
As a result, the molality of the solution will be
So, keep in mind that you need moles of solute and kilograms of solvent in order to determine molality.