Moles and molar mass
Moles are a count of particles. Divide mass by molar mass to get moles.
A worked example
How many moles in 60 g of carbon dioxide? CO2 is 44 g/mol, so 60 / 44 = 1.4 mol.
The mistake people actually make
Molar mass comes from adding every atom in the formula, so H2O is 2(1.01) + 16.00 = 18.02 g/mol.