There is ONLY one God.
This one God consists of three distinct persons:
The Holy Spirit
ALL men have sinned against God, thereby pushing themselves away from God.
note: all people have sinned except for Mary and Jesus
There is nothing man can do on his own to obtain salvation -- no amount of good works, kindness, charity, etc., is sufficient.
To obtain salvation, a person must do ALL of the following:
Admit he is a sinner
Sincerely repent of his sins
Sincerely intend to try to lead a new life without sin
"Believing in Jesus" means believing that Jesus is God, NOT just a messenger from God such as a prophet or apostle.
We are in the midst of a spiritual war but the outcome of that war is already determined. Jesus conquered death and sin for all eternity by His sacrifice on the Cross. He proved it by coming back from the dead.
THE ONLY GOSPEL (GOOD NEWS) OF JESUS CHRIST
1 Corinthians 15:1-8,11  Now, brothers, I want to remind you of the gospel I preached to you, which you received and on which you have taken your stand.  By this gospel you are saved, if you hold firmly to the word I preached to you. Otherwise, you have believed in vain.  For what I received I passed on to you as of first importance: that Christ died for our sins according [i.e., in accordance with] to the Scriptures,  that he was buried, that he was raised on the third day according to the Scriptures,  and that he appeared to Peter, and then to the Twelve.  After that, he appeared to more than five hundred of the brothers at the same time, most of whom are still living, though some have fallen asleep.  Then he appeared to James, then to all the apostles,  and last of all he appeared to me also, as to one abnormally born.  Whether, then, it was I or they, this is what we preach, and this is what you believed.
Galatians 1:6-9  I am astonished that you are so quickly deserting the one who called you by the grace of Christ and are turning to a different gospel --  which is really no gospel at all. Evidently some people are throwing you into confusion and are trying to pervert the gospel of Christ.  But even if we or an angel from heaven should preach a gospel other than the one we preached to you, let him be eternally condemned!  As we have already said, so now I say again: If anybody is preaching to you a gospel other than what you accepted, let him be eternally condemned!
Isaiah 44:6-8  "This is what the LORD says -- Israel's King and Redeemer, the LORD Almighty: I am the first and I am the last; apart from me there is no God.  Who then is like me? Let him proclaim it. Let him declare and lay out before me what has happened since I established my ancient people, and what is yet to come -- yes, let him foretell what will come.  Do not tremble, do not be afraid. Did I not proclaim this and foretell it long ago? You are my witnesses. Is there any God besides me? No, there is no other Rock; I know not one."
Mark 12:28-32  One of the teachers of the law came and heard them debating. Noticing that Jesus had given them a good answer, he asked him, "Of all the commandments, which is the most important?"  "The most important one," answered Jesus, "is this: `Hear, O Israel, the Lord our God, the Lord is one.  Love the Lord your God with all your heart and with all your soul and with all your mind and with all your strength.'  The second is this: `Love your neighbor as yourself.' There is no commandment greater than these."  "Well said, teacher," the man replied. "You are right in saying that God is one and there is no other but him."
This God is a triune God -- one God consisting of three distinct persons:
Jesus - the Son
The Holy Spirit
This is the concept of the Trinity. It is not expressly stated in the Bible but is implied by the fact that Jesus and the Holy Spirit are spoken of in combination with the Father. The three persons of the Trinity are also referred to collectively as the Godhead.
Important note: Many people get hung up on the concept of three persons -- how can one God be three persons?
You're not supposed to understand the it. The Trinity itself is a mystery.
Matthew 28:19 Therefore go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit
John 1:1-2  In the beginning was the Word [another of Jesus' names], and the Word was with God, and the Word was God.  He was with God in the beginning.
Genesis 1:1-2  In the beginning God created the heavens and the earth.  Now the earth was formless and empty, darkness was over the surface of the deep, and the Spirit of God was hovering over the waters.
Acts 5:3-4  Then Peter said, "Ananias, how is it that Satan has so filled your heart that you have lied to the Holy Spirit and have kept for yourself some of the money you received for the land?  Didn't it belong to you before it was sold? And after it was sold, wasn't the money at your disposal? What made you think of doing such a thing? You have not lied to men but to God."