For how much, you need to test the water for salinity with a K1766 kit first to see how much is in the water now. Then add about 80% that Poolmath calls for to get to your goal level. Circulate for 24 hours and test the salinity again. Then add what is needed to get to your goal level.
Yes, Wal-Mart carries Morton Pool Salt for a very reasonable price, at least in NJ. About $7 a bag. Use Pool Math to determine how much to add. It is highly suggested you add 70%-80% of what Pool Math States. Put in the shallow end and brush slowly all over the pool to mix. You do not want a high concentration of salt being picked up by the main drain (if you have one) as I feel it will rush through the pump, etc., and you want to contain the salt in the pool to mix properly. Then you will need to test with a salt test kit about 24 hours later. Then continue to add a few more bags until you get the salt at the level you need to be.
If you have been chlorinating your pool for a year or years, you already have salt in the pool. It is best to test your level first before adding anything. Also, please add your signature so we have specifics on your pool. Thank you.