As far as salt goes there are many different types and it mostly depends as to what type to where you live.
Different salt companies use different salt additives. The salt with the least amount of additives is the best way to go. You can get salt blocks and salt chips that are 99.9% salt. Most of the pellets have a higher parentage of additives.
You should see an overflow tube on the side of the brine tank. I would fill the tank to about ¾ to the over flow. Depending on the maker of the system and how much water you use (who often the softener runs) depends on how often you should add salt.
My softener will run ever 4 nights no matter if it needs to or not. 40 lbs of salt chips will last me 5 to 7 cycles. Witch for me is about 2 weeks. Mine will hold about 100 lbs of salt chips. Depending on how hard your water is and where it is coming form you should use the power clean about every 2 months for a few days. The power clean will run every night to help with the buildup that can accrue in the resin tank.
This is just basic information and personal experience. You should refer to your owner’s manual. Block salt can sometimes be cheaper. You would need to contact a farmer’s feed store for this. But it needs to be 99.9% salt. For salt blocks you should scoop out all the slat and brine (salt water) stack your salt blocks so there is a lot so surface area and add you old salt and brine back in.