What is the best way to kill and prevent a foot fungus?

Various fungicides are available. There are two important preventative measures that I recommend. First, your feet should be allowed to completely dry, at least for some part of every day. Feet can't be kept dry all the time, they perspire and they are usually covered in shoes that do not allow good ventillation, but when you get home, take your shoes off and let your feet dry. Fungus needs moisture to grow. Secondly, many people are exposed to fungal spores from other people's feet, which can happen if you are barefoot in a place where other people are also barefoot, such as an exercise or yoga class, etc. Bear in mind that other people's feet are a possible source of infection, and limit your exposure. I do not wear shoes that other people have worn, for example.