By definition, a Christian is someone who has a personal relationship with the Lord Jesus Christ. While many people will say they believe in Jesus, what they really mean is that they believe he was an actual historical figure - kind of like George Washington or Abraham Lincoln. However, simply believing that Jesus lived, and even agreeing with some of his teachings, does not make a person a Christian.
Specifically, there are five steps necessary for having a personal relationship with the Lord Jesus Christ:
ADMIT your sinful condition. "All of us have sinned and fallen short of God's glory." (Romans 3:23)
ACCEPT the fact that Jesus Christ died in your place so that you can be forgiven for your sins and receive eternal life. "Sin pays off with death. But God's gift is eternal life given by Jesus Christ our Lord." (Romans 6:23) "God loved the people of this world so much that he gave his only Son, so that everyone who has faith in him will have eternal life and never really die." (John 3:16) "God has also said that he gave us eternal life and that this life comes to us from his Son. And so, if we have God's Son, we have this life. But if we don't have the Son, we don't have this life." (1 John 5:11-12)
ASK God to forgive your sins based on the sacrifice Jesus Christ made on your behalf. "So you will be saved, if you honestly say, 'Jesus is Lord,' and if you believe with all your heart that God raised him from death." (Romans 10:9)
OBEY Jesus. "And Christ did die for all of us. He died so we would no longer live for ourselves, but for the one who died and was raised to life for us." (2 Corinthians 5:15)
GROW in your Christian walk and witness. "Let the wonderful kindness and the understanding that come from our Lord and Savior Jesus Christ help you to keep on growing." (2 Peter 3:18) (all CEV)