The relative atomic mass (Ar) of an element is the average mass of its atoms compared with one twelfth of the mass of a carbon-12 atom. It is an average because it takes account of the isotopes of the element. Ar has no units. Values you need include H = 1, C = 12, O = 16, Mg = 24, Ca = 40 and Cl = 35.5.
The relative formula mass (Mr) of a substance is found by adding up the relative atomic masses of all the atoms in its formula. For a simple molecule this is called the relative molecular mass, and the method is the same. For water, H2O, Mr = 2 × 1 + 16 = 18. For calcium carbonate, CaCO3, Mr = 40 + 12 + 3 × 16 = 100. Brackets multiply everything inside them, so Ca(OH)2 has one Ca, two O and two H. For MgCl2, Mr = 24 + 2 × 35.5 = 95.
The same method works for a whole balanced equation. Multiply the Mr of each substance by its number in the equation and add them up. In 2Mg + O2 → 2MgO the reactants add up to 2 × 24 + 32 = 80 and the products add up to 2 × (24 + 16) = 80. The totals are equal because mass is conserved in a chemical reaction.