Monkey, your question is the most important one you've ever asked. It can be answered very simply, but at the same time, it can be the most difficult question to answer. The answers above are all part of the solution.
There are several Bible verses that simply say, for example, "Believe on the Lord Jesus Christ, and you will be saved." That includes, as said above, you must believe that Jesus Christ is the Son of God AND accept Him as your Lord and Savior. Savior = He paid your sin debt, so you don't have to. Lord = He's in charge of the rest of your life. Because of His love for you, and your love for Him, you try your best to please Him, just as you try to please your wife. Because you're a human being, you'll mess up, more often that any of us like to admit. But there's grace.
Grace is a gift. You can't earn it, and you don't deserve it (and neither do I or any of the rest of us). All we can do is accept it.
Also as said above, the devil is very good at making you doubt. Asking questions is his biggest weapon. "If you're really a Christian, how can you (fill in the blank)?" Testosterone is his second biggest weapon. We (men) can't help looking at women. I heard it said that the first look is free, it's the second look (and subsequent looks) that gets you in trouble. (If that's not what leads to your doubts, you can substitute whatever temptation gives you the most trouble.)
Coming face-to-face with your own mortality can lead to such thoughts. Whether it's evasive maneuvers to avoid a collision (auto or plane or boat or motorcycle) or a phone call from the doctor's office that your test results are back and can you come in tomorrow or hearing that a good friend has died in a plane crash or that another is in ICU and about to go on the "waiting for a liver transplant" list, all trigger thoughts of eternity and where you will spend it.
You're not alone, Monkey. We all have weak spots, we all have times when the devil can get to us. That's when we all need to dive head-first into the Bible and read the unchanging, ever-true Word of God. That's when we all need to pray. That's when we all need the fellowship of our Christian brothers and sisters.