A ginger bug is the starter you can use to ferment homemade soda.
Mix 100g water, 10g sugar, 10g ginger in a jar with a lid, then give it a shake. Make sure you include the peel, as this is where the yeast and other microbes reside.
Leave in room temperature for 24 hours.
Refresh with 10g sugar and 10g ginger again, and repeat everyday for about a week or two, until it is bubbly when shaken.
It can be kept in the fridge for a week at a time, and can be refreshed with 5g sugar and 5g ginger. The ginger is not strictly necessary at this point, but it can be helpful to reintroduce any depleted microbes.