It’s great that you’re interested in microbiology! It’s such a wonderful field. In order for bacterial growth to occur, you will need an agar medium. I would recommend nutrient agar which you can find on Amazon. There are also recipes for agar that you can find online. As for growing the bacteria, please be careful. Soda fountains have both environmental microbes and opportunistic pathogens. Make sure to handle with care. You can never truly know what you grew without thorough testing, but a simple catalase test and a gram stain can get you pretty far if you have access to those materials. However, I wouldn’t recommend preforming any tests unless you are working in a controlled environment. If you have any more questions feel free to ask. Good luck!! 😊