I actually think Wikipedia provides a thorough account of what Christianity is, at least for a general, primary understanding of its tenets.
"Its followers, known as Christians, believe that Jesus is the Son of God and the Messiah (or Christ) prophesied in the Old Testament, the part of their scriptures they have in common with Judaism. To Christians, Jesus Christ is a teacher, the model of a pious life, the revealer of God, and most importantly the saviour of humanity who suffered, died, and was resurrected in order to bring about salvation from sin. Christians maintain that Jesus ascended into heaven and most denominations teach that Jesus will judge the living and the dead, granting everlasting life to his followers. Christians describe the New Testament account of Jesus' ministry as the Gospel, or "good news".
The Trinity is often regarded as an essential doctrine of mainstream Christianity. The common understanding of the Holy Trinity, espoused in the Nicene Creed, is one God who exists in three Persons – Father, the Son, and the Holy Spirit – who are coequal, co-eternal, of the same substance. "Father, Son and Holy Spirit" represents both the immanence and transcendence of God. God is believed to be infinite and God's presence may be perceived through the actions of Jesus and the Holy Spirit."
Source:
Christianity - Wikipedia, the free encyclopedia
I would personally recommend that you read through the entire New Testament (including Revelation) as well as Genesis and Exodus in the Old Testament to understand all of the main stories, but eventually the entire Bible is key. Understandably, however, the prophetic and genealogical books are more difficult, but I find that their contents only strengthen your faith once you study them. Consistent Bible study will allow you to understand the "core" better!