If you want to use essential oils for athletes foot, tea tree oil (Melaleuca alternifolia) is a top choice to fight fungus. A simple technique is to blend tea tree with a little carrier oil and apply to areas with the fungus.
If the fungus is well-established under your toenails, tea tree alone may not be enough. Try one of the following stronger blends.
From Advanced Aromatherapy:
Blend the oils in a dark bottle. Apply a few drops to fungus two times a day for 14 days. If you don't see visible improvement after 14 days, stop using this blend.
In addition to tea tree, The Complete Book of Essential Oils and Aromatherapy suggests the following essential oils for athletes foot fungus:
A blend from Hands-On Healing Remedies:
Blend all oils in a dark bottle. Shake well. Let set in a cool, dry place for 24 hours before using.
To use: Shake. Apply to a few drops to dry feet. Apply to both feet even if only one has fungus. Be sure to massage some of the oil between toes and on toenails. Let your skin absorb the oil for a few minutes before putting on socks or hosiery.
For an athlete's foot soak, blend the following essential oils in 1/2 teaspoon carrier oil:
Stir the blend into a basin of comfortably hot water. Soak your feet for 15 minutes or longer.
To make a foot powder, use one of the following recipes. Place the ingredients (starting with the dry ones) into a resealable plastic bag. Close the bag and mix well with your fingers (through the outside of the bag). Apply as needed to your feet.
Source: Aromatherapy for Dummies
Keep your feet as cool and dry as possible to create an unfriendly environment for fungus.
Consider an overall yeast/fungus eradication program — consult a naturopath or other holistic healthcare practitioner.