It is very simple: Believe that God's Son, Jesus Christ, has paid the price for your sins.
"For God so loved the world that He gave His only Son
that whosoever believes in Him should not perish
but have everlasting life."
John 3:16
A Christian believes God is God and Jesus Christ is His son who came to earth as the perfect God/human. A Christian admits he/she is a sinner in need of forgiveness before God. A Christian believes Jesus Christ paid the price for his/her sin. A Christian believes that after Jesus died on the cross for our sins, He later rose from the dead. A Christian believes Jesus now lives in Heaven to intercede for those who believe in Him and also is preparing for our arrival in Heaven.
The only thing we need to do to be a Christian is to believe and accept. No amount of good behavior and deeds will get us into heaven.