Molarity is defined as the number of moles per total liter of **solution.** So if you have a one molar solution, it is one mole of solute per liter of solution. So in regards to your question, you'd have to use both. First you'd determine the number of moles of solute you used based upon the molarity of the initial solution. Once you had determined the number of moles of solute, you'd then divide it by the total amount of solution to get your concentration.

As an example, supposed you had 436 mL of a 2.0 molar solution of CuCl_{2} and you added that to 1000 mL of water. Your first step would be determining how many moles of CuCl_{2} you have in your original solution. Since one liter has 2.0 moles, .436 liters would have 0.872 moles. With the number of total moles in your solution now known, you simply divide by the total number of liters of solution (1.436) and you'll have your solution's concentration; 0.607 Molar.

Make sense?