Answer:
First of all, you should understand that force and mass are quite different things; in many cases it DOESN'T MAKE SENSE to convert one from the other.
In the following special case, you can do a conversion: if you assume measurements near the Earth's surface, a mass of 1 kilogram is attracted with a force of 9.8 newtons. So, if you know the weight in newtons, you can divide that by 9.8 to calculate the mass in kilograms.