What are the requirements (e.g. wali) for a Muslim-Christian marriage?

I'm a Muslim, Alhamdulillah. My wife is a Christian. We got married in the court-house. Bad things started happening to us, and we thought it's probably because we didn't get married the Islamic way, making this haram technically. We are trying to correct that, and are thus looking for perform a nikah in the masjid. The problem is in finding a wali as both of us don't have our relatives here. The following are my questions: 1. Is there anything else required besides both of us and two witnesses being present there? 2. Do the witnesses have to be Muslims? 3. How much does the Imam charge for the Nikah?