Apply ice, and rest. This is important. Go easy the next day. No bumps or aggressive skiing. Have the medical clinic check your knees if they're really sore.
More severe pain requires immediate medial attention, and obviously, no skiing.
Sometimes faulty foot mechanics can create knee symptoms in skiing and a podiatrist can check that out when you get home. Rest always helps. Apply ice 5 - 10 minutes. If the injury is serious, get medical attention to prevent it from worsening.