How can I fix the damage?

Robert Jones, stop using your phone. You are running the risk of making a bad situation worse. You need to approach this like any other submersion. First thing to do is to no longer try to start or charge your iPhone. Remove the battery, and disassemble is right down to the logic board . Clean everything with isopropyl alcohol (90%+) and a soft brush, including all connectors and cable ends. Use this guide to see how to clean it. It was written for a 3G but all the points are pertinent to your phone as well. Check for any burned or missing components. You may have to repeat this step a couple of times. Once it is cleaned, reassemble it and replace the battery, then reevaluate for any real damage. The longer you wait the more corroded it will get.