The Resurrection of Jesus. That's a vital key to believing in God. If Jesus is resurrected then there is a God, and that God is the God of the Bible.
Recommended reading (definitely read the first two):
"The Historical Jesus," by scholar Dr. Gary Habermas;
"The Case for the Resurrection of Jesus," by Dr, Gary Habermas.
Both are available on Amazon.
Amazon
Also,
"The Case for Christ," by Lee Strobel"
"New Evidence that Demands a Verdict," by former skeptic Josh McDowell;
"Baker Encyclopedia of Christian Apologetics," by Dr. Norman Geisler;
The more you read along these lines, the greater your faith in God will be. I promise you. Works for me.