I'm a teenager wanting to know the truth, so please help me out. How can I be sure that the Bible is fully correct? I mean people must have added or taken out scriptures to try to confuse us. How can I know what religion is correct? I know I have to have faith, but in what? I do believe in God, but there are a lot of religions.
Faith is what you trust is true after looking at all the evidence. I've never been to Australia, but I trust that Sydney exists because I find the evidence reliable. If I ever did go there, it would no longer be a matter of faith because I would have direct evidence. We do this so often in life that we don't often think about it, except in major things.
Why do you conclude that people must have altered the Bible? What evidence leads you to that conclusion? I've looked at the evidence (see: Can We Trust the Text of the Bible?), so I'm fairly sure you don't have evidence that it was altered. In other words, you are acting on faith based on what someone else told you (faith-based on weak or non-existent evidence). I can't make you have faith in the Bible or anything else. I can only present the evidence and encourage you to trust what the evidence points to.
In the same way, I have looked at other religions. Consistently the evidence points that they are frauds.
""Present your case," says the LORD. "Bring forth your strong reasons," says the King of Jacob. "Let them bring forth and show us what will happen; let them show the former things, what they were, that we may consider them, and know the latter end of them; or declare to us things to come. Show the things that are to come hereafter, that we may know that you are gods; yes, do good or do evil, that we may be dismayed and see it together. Indeed you are nothing, and your work is nothing; he who chooses you is an abomination" (Isaiah 41:21-24).