I wish I had a magic cure for you at home but I really think you cat needs to be seen by a veterinarian.  Puss usually signals infection--your cat may have an abscess or abscessed mammary gland.  If your cat has had a recent litter or heat cycle the mammary gland may be abscessed.  Indoor/outdoor cats that fight with strays or neighborhood cats are prone to developing abscesses.  Occasionally indoor only household cats can develop abscesses from fighting.
Mastitis (inflammation of the mammary gland) or cancer of the mammary gland can cause a secondary infection. 
An examination and fine needle aspirate will be done to see if it is infection or if there is another cause for the discharge.
Antibiotics may be needed if it is an infection.  If your veterinarian has concerns that it may be cancer a biopsy or mass removal may be needed.
Before your appointment warm compresses the area may bring some relief to your cat.
Contact your local shelter to see if they run a low-cost or income based clinic.  They may also know of a clinic that is low-cost, income bases or that takes payments.