Percentage composition is easy: divide mass of the elements over the mass of the compound and multiply by 100.
Empirical formula: take the masses of the elements and divide by their molar masses (given by the periodic table) to determine how many moles there are of each element.
Then, divide both elements' number of moles by the smallest of the two to get approximately whole numbers.
Once you have approximately whole numbers (if they aren't, you can multiply them to scale up e.g. 0.5 Fe, 1.5 O -> 1 Fe 3 O, therefore FeO3) you will be able to write the empirical formula.
Additionally, for future reference, to get the molecular formula, you have to know the molar mass of the molecule and the molar mass of the empirical formula.
Divide the molar mass of the molecular formula by the molar mass of the empirical formula to get an approximately whole number, which will function as a multiplier.
Multiply the empirical formula and you will get the molecular formula. (e.g. CH2O = 30g/mol, molecular formula = 60g/mol -> 60/30 = 2, therefore CH2O becomes C2H4O2 (Acetic acid)).