Firstly:
http://www.light-links.net/2008/09/conventional-versus-religious-logic.htmlYou will need to start out by agreeing on a definition of "God" for the purposes of the debate. For instance:
1. God is all-powerful
2. God is all-merciful
3. God created everything
4. God knows all, the future, mens thoughts, ect...
5. God listens to the prayers of humans and will sometimes interfere in human affairs
6. God punishes or rewards people with heaven or hell upon their deaths
Then you could argue something like: "It is more rational to disbelieve in God than to believe in God."
You could add some Islam-specific things like:
1. Mohammad is God's final prophet
2. The Quran is the direct, unaltered word of God
3. Mohammad's example (from the Hadith) is a model of morality for all people in all times
And argue something like: "It's is more rational to disbelieve in Islam than to believe in it."
Of course, then you'll need to agree on a definition for "rational"!