Idonjohn's website that he mentions is excellent. I have been to it a number of times and have tried to follow it's advice. I have also found a tract based on writings by Josh McDowell to be helpful. The tract is called "In search of truth" and can be ordered from
www.crossway.org. This tract pretty much wraps up with the following:
"I prayed three things that night to establish a relationship with the resurrected, living Christ, who has since transformed my life. First, I said, “Lord Jesus, thank you for dying on the cross for me.” Second, I said, “I confess there are things in my life which are not pleasing to you. I ask you to forgive and cleanse me.” (The Bible says, “Though your sins are like scarlet, they shall be white as snow,” Isaiah 1:18.) Third I said, “Right now, in the best way I know how, I open the door of my heart and life, and I trust you as my Savior and Lord. Thank you for coming into my life by faith.”
"After I prayed, nothing happened. There was no bolt of lightning. I even said to myself, “Oh no! What’d I get sucked into now?” I felt I’d gone off the deep end. And some of my friends agreed. But I can tell you now that in the months following, I found that I had not gone off the deep end. For one thing, I had finally found mental peace. I had also found control of my temper. And old hatreds were gradually turning to love."
If you realize Christ is trustworthy with your life and you sincerely pray to Him something like the above, He will save you. If you doubt afterwards, you can know that it only takes a moment to be saved and that moment in the past when you trusted Him is when you got saved. After you are saved, you cannot be kicked out of the Kingdom of Heaven, no matter what!