Hello Phoebe,
The best way to remove sodium and excess minerals is to filter your water with a reverse osmosis system (RO). This system uses a membrane with pores small enough to only allow water molecules through, while leaving minerals behind. Using RO is a good idea regardless if you high high levels of sodium because excess metals in the water can detrimentally interact with molecules in soil and water based systems. However, due to the removal of minerals, the waters buffering capacity is reduced; therefore, the pH of a system can be more prone to fluctuations.
Another way to reduce the sodium in your water is to use potassium chloride (KCl) rather than sodium chloride (NaCl) when treating. This will incorporate potassium rather than sodium, which is a macronutrient for plants.
Hope this helps!