There is a cut on my leg and a ring of pus spreading from it. I am a diabetic smoker. What should I do?

Infection. This is an infection and needs to be treated with antibiotics. Which antibiotic depends on the circumstances surrounding the injury and your health. Please seek immediate medical attention.
Go to ER. You are at risk for periferal artery disease. You may have an active infection. Better not delay care.
See a doctor. As a diabetic, you are more prone to developing certain infections. The lower extremities are a prime site for skin infections, when left untreated, to potentially become deeper (e.g. Cellulitis) & more serious. See your physician.
GO TO ER. This needs evaluated asap. You may need surgical drainage. You need antibiotics and wound care. You need further work up to see the extend and depth of infection as well as have your circulation tested to see if you have enough blood flow to heal. The sooner you get treated the more likely you can avoid complications.