I empathize with your question about proof for God's existence. I personally find the strongest proof that there is one God who created the world, took the Jewish People out of Egyptian slavery and gave us the Torah at Mt. Sinai to be the proof from Maimonides. Look at the way in which the Torah was given to an entire nation "in person" (Deuteronomy 5:4) - a Torah that teaches about the Creation along with everything necessary for life in this world and the next.
The Sinai experience has been passed down from father to son, from generation to generation until today. It is the same history and teachings that are transmitted yearly at the Passover Seder, and daily in study halls throughout the world. This is the testimony of an entire people to the truth that they experienced thousands of years ago that there is one God who created the world, Mankind, gave us the Torah and is eternal.
I suggest reading one of the classic books of Jewish philosophy called the Kuzari which is a book written almost a thousand years ago by Rabbi Yehudah haLevy in the form of a dialog between a Rabbi and a Gentile king. It is theological debate between a Greek philosopher, a Christian, a Muslim and, finally a Jew. It is truly fascinating. There is an excellent new translation (with footnotes) by Feldheim Press that ought to be easily available over the Internet.
Allow me to also suggest that you try a Gateways retreat (preferably over a Shabbat, but failing that start with a lecture or two). The classes and the retreats offered by Gateways are excellent and cater to all ages and religious observance in an appealing and non-threatening. You can try looking at what they have to offer on their site: www.gatewaysonline.org
