This is the first chapter in the book we use in first grade, and it's the first question we ask the kids.
We usually get some pretty random answers, but "love" always figures in. And that's true.
The official answer?
God created me to know him, love him, and serve him in this world, and be happy with him forever in the next.
That's it. That's why God made you.
He made you to know him: to realize He is God; to know about His Son, Jesus, and His work of Salvation; to know Him in his church and in His sacraments.
To love him: To pray to him, to give him the devotion he is due, to follow his commandments.
To serve him: To "be his body", as St. Terese of Avila wrote. To serve our neighbors, to demonstrate Christ's love to the world, and to do the work he gives us to do well and cheerfully.
That's the basics.
Be happy with him forever in the next: To live with him in Heaven forever.
Our goal in life? To be saints. (Everyone in Heaven is a saint. Side note: dead people are NOT ANGELS. If they are in Heaven, they're saints. ANGELS ARE DIFFERENT!)
We get to be saints by knowing, loving, and serving God.
All of the Church's doctrines, sacraments, rites, rituals, ceremonies, and hierarchy as designed to help us achieve this goal.