Welcome, @Maytoday!
Only one website matters, the Church's official one. Whatever you find elsewhere may or may not be correct or consistent with what the Church teaches.
The 13 Articles of Faith might be a good place to start with what we believe - they were written by Joseph Smith, through whom the Church of Jesus Christ was restored.
As @Grunt said, the Book of Mormon is another good starting point (and, there's an app for that ).
What's to argue? We believe what we believe. We believe it because it is revealed by God through prophets and apostles. Whether you understand it or see reason or logic in it won't matter - we believe what we believe. People have been trying since the 1800s to "argue" us out of our beliefs - doesn't work. So what is there to argue about? If you don't understand what we're saying, OK, we'll try to explain it in different terms. If you don't believe the things we believe, that's your right and choice, but there's still nothing to argue about.
How exactly is a religious document "proven true"? How do you prove that Jesus Christ is the son of God? How do you prove that he atoned for our sins and was resurrected? How do you prove that God is real? You can find our beliefs in our scriptures. Every April and October, the leaders of our Church expound on what you find in said scriptures - helping us to apply these things in our lives - in a meeting we call "General Conference". But you're not going to find us citing a creed or whatever.
If you want to take a deep dive into early Church history, you could try the Joseph Smith Papers website.
Other than all that, it might be best if you just start with whatever simple question you might have, and we'll have a go at answering.
I will say that in my experience, this Third Hour forum is the one place on the internet where you will find faithful members of the Church who support the leaders of the Church (as opposed to many other forums where you'll find a mixture of that, wolves in sheep's clothing, people who want to destroy the Church, and heaven knows who else).
Finally, perhaps @prisonchaplain can give you some tips on engaging with latter-day saints. (Should he have time and notice this.)