Molarity is a measure of moles per liter. So you need to set up the equation as [m] = moles/liters. So if you need a 10 ml solution with a .4m concentration you need to fill the equation in. You know you need [m] to equal .4 and you know you need .010 liters. So solve for moles. .4 = x/.01 == .4*.01=.004. So you need .004 moles of your molecule. A mole is 6.022 x 1023 (602200000000000000000000). So next what you need to do is find the molar mass of you molecule which is found of the periodic table. For example H2O2 (hydrogen peroxide) would have a molar mass of O=16 H=1 so ((16*2)+(1*2))= 34 grams a mole. Once you found how many grams a mole your molecule is then you times it times your wanted moles, in your case .004, and you have how many grams you need to put into your solution.