The easiest way to do this is by counting up the total number of atoms you have on each side. Then you can change the stiochemetric coefficient infront of each of the compounds so that the number of atoms are the same on both sides.
E.g. Your first question: Na + I2 -> NaI
On the reactants side you have 1 atom of sodium and 2 atoms of iodine, on the products you have 1 atom of sodium and 1 atom of iodine. To correct this imbalance you must place a coefficient of 2 infront of NaI and a 2 infront of Na so you have:
2Na + I2 -> 2NaI
2 atoms of sodium in reactants, 2 atoms of iodine in reactants, 2 atoms of sodium in products and 2 atoms of iodine in products. These problems just take a lot of practice.