One, of course, begins the journey of "believing in God" by taking God on faith. But this is asking no more than asking a 5-year-old to believe in the existence and validity of those subjects one begins to learn in first grade. The child has to take their mere existence — and the teacher's competence — on faith.
After one has learned the rudiments of spelling, arithmetic, geography, vocabulary, etc., somewhere between the ages of 10-15, one begins to add personal experience to the a prioris first accepted by trust, and thus the emerging adult rounds out the reality of these subjects for themself.
The same is true with faith in God. As one ages, and (hopefully) has more and more direct experiences with issues resolved by "chance vs. heaven," only then does one begin to SEE (in a logical sense) the existence of God.
If you are someone who WANTS to believe that God is more than a "fictional existence," I recommend you get started on your own journey right away. Some of the best things in life really do take time to understand, or to appreciate.