Swelling and pain in the penis after first-time sexual activity is often due to minor injury, friction, or soft tissue strain, especially if there was prolonged or vigorous activity, and this usually improves with simple care. You should rest, avoid sexual activity or masturbation for a few days, wear loose-fitting underwear, and apply a cold compress (wrapped in cloth) for short periods to reduce swelling. Mild pain can be managed with over-the-counter pain relief if needed, but avoid applying any creams unless prescribed. However, it is important to watch for warning signs such as increasing swelling, severe pain, difficulty passing urine, redness with warmth (suggesting infection), or a bent or deformed erection (which could indicate a rare condition called Penile fracture), in which case you should seek immediate medical care. If symptoms do not improve within a few days, consult a doctor for proper evaluation and treatment.