I have a rash on the inside of my foot could you tell me what it is?

It could be...: Athlete's foot. The best otc medication (in my opinion) is lotrimin (clotrimazole) ultra. Dilute vinegar soaks are good as well. Dry the inside of your shoes by placing them up-side down over an air return vent at home overnight. If this doesn't make things better in a couple days, see a podiatrist.

Eczema or fungus?: Is it on both feet? Is it itchy? May be a contact dermatitis or it might be a fungal infection. See you dermatologist and have a culture obtained. This way the correct treatment can be started.

Without seeing it: It is very difficult. Rashes have different patterns and characteristics, colors etc which give clues. Example, are there blisters? Renews? Is the redness in a certain shape etc? More clues or hints are needed. Otherwise it can be anything from poison ivy, to athletes foot, to allergic reaction etc.