Avogadro's number is a constant value equal to the number of molecules in one mole. Specifically, it is equal to the number of atoms of 12g of carbon-12. A single mole of any substance is always equal to that amount of molecules. Figuring out the number of molecules a substance has when you only know the number of moles is a straightforward process. A mole is the amount of one specific element in its pure form.
Record how many moles you have and the substance with which you're working. For example, if you're working with H2O, your recording might look like this: 4 mol H2O.
Multiply the number of moles by Avogadro's number. The example would look like this: 4 mol H2O x 6.02 x 10^23.
Write down the answer which, in this example, is 24.0x10^23. Simplify the result into a more basic number by sliding the decimal to the left one spot. The example now becomes 2.4 x 10^24. The exponent becomes 24 because you've gone from 24 to 2.4. Therefore, you added another power of 10 to the scientific notation.