You need dishwasher salt and there should be a reservoir for it, the instructions will tell you about it. I use whatever tablets are cheapest when I buy mine, the powders and liquids are messy. I do find that rinse aid (the ones that say they dry plastic) make a difference with (ha) drying plastic and preventing watermarking. You will find it is a necessity now you have one!!
If you live in the south, think about getting a water softener installed. They aren't cheap but last ages and are seriously life changing.