"Why do I need to believe in Jesus since I already believe in God?" That is an excellent question, indeed!
The simplest answer I can offer is that Jesus taught that we cannot have a relationship with God the Father unless we have a relationship with God the Son. Jesus said, "I am the way, the truth, and the life: no man cometh unto the Father, but by me" (John 14:6). Earlier, Jesus also said, "If God were your Father, ye would love me: for I proceeded forth and came from God; neither came I of myself, but he sent me." I'm sure you'll agree that this raises an important question: Why did the Father send Jesus?
The Bible teaches that we've all sinned against God. The Bible says, "For all have sinned, and come short of the glory of God;" (Romans 3:23). The Bible also teaches that our sin separates us from God and makes it impossible for us to have a relationship with Him. "For the wages of sin is death; but the gift of God is eternal life through Jesus Christ our Lord" (Romans 6:23). Therefore, we have a problem that needs a resolution.
We understand from Scripture that God has always required a blood sacrifice to cover our sins. Hence the bloody animal sacrifices carried out by the Jewish people throughout the Bible in the Tabernacle and the Temple. We see that God's plan was for the Jewish animal sacrifices to forshadow the coming of Christ, the Son of God -- the ultimate sacrifice. In fact, the Bible teaches that God had worked out this plan even before he created the world. "Forasmuch as ye know that ye were not redeemed with corruptible things, as silver and gold...But with the precious blood of Christ, as of a lamb without blemish and without spot: Who verily was foreordained before the foundation of the world, but was manifest in these last times for you" (1 Peter 1:18-20).
Consider how the Biblical Jewish Temple and Tabernacle picture Christ. They both had one door leading into the outer court (symbolizing the idea that there is just one way to God). Also, they both had an alter (where bloody animal sacrifices were performed) between the door and the place of God's special presence -- picturing the idea that God can only be approached as the result of a blood sacrifice. We understand that this design was God's way of preparing hearts for the idea that Christ would come as the "one way" and the final, perfected blood sacrifice through which we may have access to God the Father-- and a real relationship with Him.
The good news of the New Testament is that Christ did come to die on the cross as the final payment for all our sins -- fulfilling all the prophecies of the Messiah in the Jewish scriptures. Jesus Christ came as the perfect sacrifice that satisfied God’s demand for justice against our sins. Because Jesus came as God in the flesh and allowed his blood to be shed for us, eternal life is offered to us as a free gift. God paid the price for our sin Himself! And Christ rose again from the dead, too!
Our responsibility in all of this is to choose to place our faith in Christ for forgiveness of our sins. "For whosoever shall call upon the name of the Lord shall be saved." (Romans 10:13) "For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life." (John 3:16). The word "believe" in John 3:16 means to place your trust in Christ. God promises eternal life to anyone who places their trust in Jesus Christ alone.
Jesus taught that "...except ye repent, ye shall all likewise perish." In calling us to repent, he calls us to make a conscious decision to leave behind our old sinful ways and to turn to him. Importantly, the Bible teaches that there's nothing that we can do to earn forgiveness of our sins. Christ has already done all the work. We simply need to receive him. The Bible says, "For by grace are ye saved through faith; and that not of yourselves: it is the gift of God: Not of works, lest any man should boast (Ephesian 2:8-9).
When you're ready to repent, you may receive Christ as your personal savior by praying a simple prayer like this:
Dear Lord Jesus Christ, I acknowledge I have sinned against a Holy God and that my sins have separated me from you. I believe that you, Jesus, are the Son of God, and I believe you shed your blood and died to pay for my sins on the cross. And I believe that you rose from the dead. Today, I repent and place my full dependence and trust in You for my eternal salvation. Please help me to live for you because you have loved me so very much.