I have pus formed inside the thick skin on the base of my foot. Who can help?

Many choices. Your primary care doctor, an urgent care or walk-in clinic, or a podiatrist can all examine and treat, including removing callus and/or draining an infection if necessary. If you are diabetic, have poor circulation or there is a very deep/complicated/badly infected wound, you may need to see a podiatrist or general surgeon, but even then primary or urgent care can get you started.