You likely have a fungal infection such as tinea cruris (jock itch), which commonly occurs in warm, sweaty areas. Apply an over-the-counter antifungal cream (such as Clotrimazole, Miconazole, or Terbinafine) twice daily to the affected area and nearby skin. Keep the area clean and completely dry, and wear loose, breathable cotton underwear. If the infection does not improve within 1?2 weeks or continues to spread, consult a dermatologist for further treatment.