To do this, you need to know what a mole is and how to determine the molar mass of a substance. This page gives a good but brief introduction:
http://www.ilpi.com/msds/ref/mole.html
Then for any given substance, you need it's chemical formula and you need a periodic table (Google -- seberal good ones on the web) to determine the atomic "weights" (masses really) that need to be added to determine the molar mass. This will be the mass (in grams) of 1 mole of the substance (the same numeric relationship will exist between milligrams and millimoles, or between micrograms and micromoles).
For your specific question, 1 mole Na = 22.99 g
and 1 mole K = 39.10 g