I have a sciatic nerve injury with pain in the left buttock. what can i do?

You have to see what: Is the cause of it , most likely disc problem, go and see your physician he will do x-ray and possible MRI to diagnose the reason.

Sciatic nerve: The sciatic nerve primarily supplies function below the knee in the calf and foot. Primary buttocks pain without distal radiation may indicate a myofascial process rather than a nerve injury.