You have prayed the "Sinner's Prayer," which is a term used by the church that means you have repented and asked Jesus to come into your heart and take charge of your life. It doesn't matter if you repeated the words after someone at an altar call, or if you spoke them out of your own heart, in complete privacy. He heard you! You have made the transition into a whole new way of life and a new beginning. You are now in Christ!
2 Corinthians 5:17
"Therefore, if anyone is in Christ, he is a new creation; old things have passed away; behold all things have become new."
You now have a clean slate! The past is forgiven and your future is in God's hands! If you will just trust Him, you will be fine. Sounds easy, doesn't it? What? You mean you don't know how to do that? Don't panic. God doesn't expect you to be able to do it perfectly at this point in time. It is a process. And you won't be alone as you progress toward maturity in your new life. He is with you always. I am not saying that you will never have a problem, or have to face any struggles as you follow Him. I am only saying you will not have to face them alone! He will help you, comfort you and strengthen you for those struggles. The Word refers to it as an "exercise unto Godliness." It doesn't happen overnight; but day by day, you will find yourself growing stronger. As I said before, it is a process. Be patient. He is.
Study the Word. Learn all you can about Him. Reading how He interacted in the lives of the people in the Old Testament is a good place to start. Some of it will be reminiscent of the Bible stories you heard as a child. They are much more than stories. They are God's dealing with people just like you and me. You will learn some of the many ways He showed Himself strong on behalf of those who were obedient to Him; ways He dealt with those who were not, and the faithful fulfillment of the promises He spoke to those who would heed His commands. He shows Himself to be trustworthy.
Next stop...New Testament. Read the Gospels. Allow these books to introduce you to Jesus Christ; God incarnate. Again, study Him. See His interactions with the disciples, the religious leaders, and the ordinary people. Understand the events leading up to the crucifixion, death, burial and resurrection of Jesus. Did the things spoken by God, and through Jesus, come to pass? Was He faithful? Can you trust in the absolute truthfulness of His Word? Yes
As you walk with Him, He will create a "history" of faithfulness to you, just as He did the people you have read about. You will grow from faith to faith; as you study and trust in Him.