You're wanting to figure out the molar mass of H2O2, which tells you the mass of one mole of this compound. There are 2 hydrogen (H) atoms and 2 oxygen (O) atoms in one molecule of H2O2, so you have to add the masses of these 4 atoms.
Different elements have different molar masses. When you look at hydrogen (H) on a periodic table (if you looked up "periodic table" and hover over H with your cursor), there will be a number, 1, that shows how many protons an atom of hydrogen has, and there will be another number (that's relevant to this problem), 1.0078 (or rounded to 1.008), that shows the mass of an atom of that element. Looking on a periodic table, hydrogen has a molar mass of 1.008 and oxygen has a molar mass of 15.999.
To figure out the molar mass of H2O2, you add two hydrogen atoms, which each have a mass of 1.008, with two oxygen atoms, which each have a mass of 15.999.
2 (1.008) + 2 (15.999) = molar mass of H2O2