Pray for the gift of faith! Faith is “believing what we can’t see”, and if you keep reading your Bible and other books about Christianity, go to church so you can learn more, find friends who are also trying to live their faith and who you can “chew on” these things with, and if you keep asking God to help you grow, He’ll answer your prayers!
Mostly though, remember that even the greatest Saints doubted at times, that is normal and part of our humanity, so don’t beat yourself up about it. Rest in Him and know that He sticks with us no matter what we struggle with. If I find myself in times of doubt, I’ll pray a simple prayer from St. Thomas in the Bible (“doubting Thomas” – yep, that’s from the Bible!): “I believe Lord…help my unbelief.” (See the next question that touches on this topic, too.)