If you want to start a Walstad type el natural tank, you won't need any chemicals or supplements. But, if you want to start a high light tank, with pressurized CO2, you will want potassium nitrate, mono potassium phosphate, a trace element mix, such as CSM+B, a GH builder, such as Seachem Equilibrium, and perhaps some Excel. In between those extremes are several other options, including using a nutrient rich substrate and little or no water fertilizing.
First, decide if you are willing and able to provide the maintenance a high light tank needs, and if you want to try that. Or if you want something that is low maintenance, where you spend more time just enjoying the tank and not constantly working on it.