The answers to those questions lie in the gospels, accounts of the life and teachings of Jesus written down by eyewitnesses within the lifetimes of other eyewitnesses. To know Jesus, you should read the accounts we have of him; the gospel of Mark is the most concise and fast-paced. (Link is for The Message version.)
But it takes a while (at least a couple hours) to read the gospels. Moreover, as accounts written two thousand years ago in a totally different cultural context, the gospels can be seriously disorienting. It doesn't work well to read the gospels trying to quickly extract principles and instructions of Christianity. The gospels are not full of convenient lists of ideas, because Christianity is about a Person (God), not an idea, and the proper "container" for a person is a story, not a list. While incomplete in itself, a list or summary can be a helpful orientation before (or during or after) reading the gospels themselves. For this, a creed is helpful.
So, for the person interested in a summary of the essential points of Christianity, here follows a discussion of the Nicene Creed, which summarizes the major doctrines of Christianity* in three paragraphs. But be prepared: Even the creed is more like a story than a theory.
Here is my paraphrase or adaptation of the story/list that is the Nicene Creed (Brackets indicate points that are not directly in the Nicene Creed), expanded into six paragraphs:
(1) From the beginning of time, before there was anything, there was God, the Almighty. God created everything else that is: the physical world and the spiritual world. [One of God's creations was humans, whom He created to love and serve Him, but who rebelled against Him in mistrust. Ever since the first rebellion, humans have lived in a broken world and in broken relationships with each other and with their Creator and Father, God.]*These doctrines are all accepted by the Catholic church, most Protestant churches, and the Eastern Orthodox church. (Where did all those different churches come from, you ask? See part 3...)
(2) For the sake of us fallen people, God sent down to us His Son. This Son is not a biological son, or in any way less than God as Father, but, mysteriously, has always existed (rather than having been created) and is one Being with God the Father and is just as much God as the Father is, so that the Father and Son are distinct but unified, two Persons but one God. This Son, Jesus Christ, took on a human body and being to walk among us and live a human life but a perfect life. He started where we all start, as a baby. He was born to Mary, a virgin, having been conceived by the Holy Spirit instead of by physical means. [He went through childhood and adolescence and grew up. As an adult, he traveled around Israel with a band of students (disciples), ministering to huge crowds by teaching about God's kingdom, healing the sick and forgiving sins. Jesus' teaching and claims infuriated the religious authorities of Israel, who conspired against him.]
He was tried in a Roman court under Pontius Pilate, who sentenced him to death by crucifixion: being nailed to a cross and dying the most shameful death available. The human agents at the time didn't realize it, but they were fulfilling God's plan for the world, the purpose for which Jesus, God the Son, had come into the world in the first place: dying to save us from the power of sin and death.
And so Jesus, the Son of God, died and was buried--and then on the third day after his death, He rose again to life. This fulfilled the prophecies in the Jewish Scriptures and in Jesus's own ministry. After rising again to life, He returned to Heaven in glory, where He has taken up his position of authority, at the right hand of the Father. He will come again to Earth, at an unknown time, for the final judgment and the end of time and the restoration of the world.
(3) In the mean time, followers of God are helped and guided by the Holy Spirit, who is the third Person of God, of one Being with the Father and Son, and as such is worshiped and glorified as God in the same way that God the Father and God the Son are worshiped and glorified. The Spirit speaks to God's followers today, as He spoke through the prophets whose words are recorded in the Bible.
God's followers form the Church, which God has made holy.
Despite the divisions and brokenness in the human institutions that represent the Church, there is one unified and holy Church which the many churches strive to manifest and to become. Also, despite the various manifestations and interpretations of the sacrament of baptism, there is one baptism which these different versions all represent. Baptism itself is connected to forgiveness of sins. Members of the Church live in hope, believing God's promise that He will restore life to the dead and bring about a new and fuller life in a new and restored world.