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 pure substance is always equal to that number of molecules. Figuring out the number of molecules a substance has when you only know the number of moles is a straightforward process.
TL;DR (Too Long; Didn't Read)
To convert the number of moles of a substance to the number of molecules, multiply moles by Avogadro's number, 6.022 × 1023.
Determine Number of Moles
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 by Avogadro's Number
Multiply the number of moles by Avogadro's number. The example would look like this: 4 mol H2O x 6.02 x 1023.
Adjust Scientific Notation
Write down the answer which, in this example, is 24.0×1023. If necessary, simplify the result into more formal scientific notation by sliding the decimal to the left one spot. The example now becomes 2.4 × 1024. The exponent becomes 24 because you've reduced the main part of the number (mantissa) by a factor of 10, from 24 to 2.4. Therefore, you added another power of 10 to the exponent part of the scientific notation.