What is Christianity, anyway? Although you can approach this question from several angles, try this one: Christianity is a way of understanding our lives, other people, and the world around us. It is a unique way of understanding these things, and it claims to be the only correct way to understand these things.
Christianity starts with a being, God, who is eternal, unchangeable, and uncaused. Everything else originates from God. God is not just a force or a principle, but a person. (In fact, God is one being which is also three persons an unusual arrangement for we humans accustomed to a one-to-one correspondence between being and persons, but not as incomprehensible as many like to think!) This God created everything (except Himself, of course).
Human beings are the crown of His creation. Though we are not "little gods" we do share some (though not all) of the characteristics of God. Key among these is the ability to make choices. According to the Christian understanding of the world, human beings choose to rebel against the Creator, something the Creator cannot tolerate and remain true to Himself.
To remedy this situation, one person of the three that are the one God became a human being named "Jesus." The primary purpose of this was to die willingly in our place to pay the penalty for our rebellion against God. Realizing that these are rather extravagant claims, God raised this Jesus from the dead in order to prove just Who and what He is.
The discerning reader will immediately note that many things are implied in this way of understanding reality, most of which we cannot explore here. But a few important points are these: our lives have ultimate meaning, we have ultimate responsibility, and there is a way available to us by which we can be on the proper terms with God, our Creator.