What is a Christian?
In the simplest sense, a Christian is any person who sincerely and completely follows the teachings of Jesus Christ. (Who is Jesus?)
In practical terms, that means the person fully believes that Jesus is who he claimed to be, and did what the Bible claims he did. (Many of these claims in the Bible about Jesus, are also shown to be true by historical evidence completely external to the Bible. More information here.)
That includes believing that Jesus is God, that his purpose on Earth was to save everyone who wants to be saved (what is Salvation?), and he did that by voluntarily and purposefully dieing on a wooden cross, thus taking on himself the punishment for sin, which is death. (Why/how is death punishment for sin?). He died so that his followers don't have to die.
Being a Christian also includes believing that Jesus rose from the dead three days after being executed by Roman soldiers (exactly as claimed by both the Bible and other historical sources external to the Bible), thus demonstrating that he has authority and power over death itself, and demonstrating that he will also raise from the dead anyone who completely trusts him to do that.
This firm belief that Jesus died for your sin, and that he was raised from the dead so that you will also be raised from the dead, is often referred to as "faith". Everyone who puts their "faith" in Jesus, will be raised from the dead and go to heaven.
This also implies that those who do not put their faith in Jesus, will not go to heaven, but rather will go to hell. (Is hell real?)
This salvation cannot be bought, earned, or deserved in any conceivable way. Salvation is a free gift, offered only by Jesus to everyone who wants to accept it. Anyone at all can receive this salvation, no matter what they have done on the past, by simply telling Jesus that they want it, sincerely, and want to follow him and everything he taught.
This act of believing in Jesus, and receiving salvation, also requires something the bible calls "repentance". That doesn't just mean that you feel sorry for bad things you have done, nor that you simply admit doing bad things. True repentance also implies changing your ways: in other words, not "sinning" (what it sin?) any more. No loner doing the bad things you were doing. The "repentance" that Jesus asks of Christians is to completely turn away from "doing bad stuff" in your life, and instead only do "good stuff". The Bible provides many examples of what God considers "bad stuff" and what he considers to be "good stuff", so there's no confusion.
So, becoming a Christian requires believing that Jesus is God, that he died for you personally, that he was raised from the dead so you can be too, and that you make changes in your life such that you stop doing the bad things that God hates, and start truly doing what Jesus taught his followers to do, which is the real meaning of "repenting". That's what it means to be a Christian.
Christians try to be more like Jesus Christ, and do what he said to do, trusting in him completely to help them do that, and to take them to heaven when they die, as promised in the Bible.