A basic bath fizzy includes citric acid, baking soda, kaolin clay, fragrance/essential oil, witch hazel and some sort of colorant. The combination of the citric acid and baking soda, when introduced to water makes amazing fizz. Mixing 1 part citric acid with 2 parts baking soda should give you the right proportion, but do test so that you can find the perfect one for you!
Here is my basic bath bomb recipe:
1 cup citric acid
2 cups baking soda
3 tablespoons kaolin clay
18ml skin safe fragrance/essential oil
Colorant (labomb colorant works great)
Combine all the above ingredients and spritz with Witch Hazel. Smash into a mold and let dry for a few minutes. Then unmold and let dry further.
Links to Bath Bomb tutorials: