For that size of tank you'd want to use 8 tablespoons of salt - dissolve it into a small bit of water before you put it in the tank too. It really doesn't matter what salt you use. Table salt will work, even if it is iodized. The iodine concentrations in table salt will not be harmful to your fish over the short term. Whatever salt you use be sure to be diligent with your water changes when the treatment is over, so your salt levels get back to normal & all traces of medication are removed (putting carbon in your filter will remove the medication too).
And no, I wouldn't say doing a complete water change is necessary, in fact it'd probably do more harm than good. Two 75% water changes on consecutive days would be much better.
Also, only stop treatment three days after the white spots on your fish go away to make sure all traces of the disease are gone.