If you were brought up as a Christian, you would learn about God from a very early age. You would be baptised and to keep the promises they made at your baptism, your parents would probably teach you prayers as soon as you could talk.
Next you would go to Sunday school where you would learn more about God and how he made and looks after you. You would say prayers to God thanking him for looking after you and so it would seem natural that you would believe in him.
It would be natural to read the Holy Bible, and through it believe in the existence of God.
You would also be taken to Church by your parents on Sunday and also at special festivals such as Easter and Christmas. At the Church you would hear people talking about God and you would assume that God exists.
All these teachings would be confirmed when you started school through such aspects as assemblies, home-time prayers, and RS lessons when teachers referred to the things you had heard at home and at Church.
When you were 12/13 you would get Confirmed at Church (Confirmation), at the ceremony you say the promises which were said on your behalf when you were baptised as a baby. Confirmation is another stage when you acknowledge the existence of God in your life. You need to attend first a course of preparation which covers all aspect of Christian beliefs.
Christians will also use the Holy Bible as part of their upbringing, helping them in their daily life to follow the teachings of Jesus. Christians also pray at home, this may cover aspects such as Thanking God, praising, asking for forgiveness, asking for yourself and asking for others.
Through prayer you are therefore having a personal relationship with God which strengthens your belief in him. Some Catholics might also have a small shrine at home with a statue to help them concentrate whilst they pray. You would go to church every sunday, which would help strengthen your belief in God.