Hello Margaret. To fix your salty soup, you can dilute it by adding more unsalted liquid, broth, or cream, or add a raw peeled potato which will absorb some of the excess sodium during simmering.
Regarding the salt conversion, table salt is much denser and finer than kosher salt. Because the crystals are smaller, a teaspoon of table salt contains roughly twice as much sodium by volume as a teaspoon of coarse kosher salt. Therefore, a 1:1 substitution will always ruin a dish. For future reference, use half the amount of table salt when a recipe calls for kosher salt, or weigh them on a kitchen scale for accuracy.