Introductory Christianity

God is invisible to the physical eyes but His wonders are noticeable even to those who disbelieve His existence. God does exist whether some people believe or not but who exactly is God? What is the need to care about the existence of God? Christianity is based on the idea that God exists and sent His Son Jesus to save mankind but is Jesus God also? Who else is God? Christians talk about the Trinity but is the idea of Trinity Biblical? Who qualifies to see God and when?