The density of something is found by dividing the mass of the object by the volume. This tells us how much mass is in unit area of the object. However, if you need to find the mass from the density, you can easily rearrange the equation. Now, you need to multiply the volume of the object with its density.
This will give you the total mass of the object. Calculating the volume is easy enough. You can measure it, if it is a geometric object. If not, you can calculate it by dipping it in a measuring cylinder with water and then calculating the difference.