Why do you believe that Joseph Smith was even telling the truth? Or, more specifically, why do you think what he was saying was true? Just because someone says something and they believe it with all their heart doesn't mean it is so.
If I had a dollar for every time someone asked me this question, I'd be rich.
I'm going to be completely candid with you, and say that like many other religious people, I inherited my religion from my parents. That is not to say that it was forced on me or that the only reason I am Mormon is because my family is Mormon. I believe in it independently. But I was not converted.
And once again, Joseph Smith was not the only one who saw the Plates. Eleven others saw them, and swore as much.
The Testimony of Three WitnessesBe it known unto all nations, kindreds, tongues, and people, unto whom this works hall come: That we, through the grace of God the Father, and our Lord Jesus Christ, have seen the plates which contain this record, which is a record of the people of Nephi, and also of the Lamanites, their brethren, and also of the people of Jared, who came from the tower of which hath been spoken. And we also know that they have been translated by the gift and power of God, for his voice hath declared it unto us; wherefore we know of a surety that the work is true. And we also testify that we have been shown the engravings which are upon the plates; and they have been shown unto us by the power of God, and not of man. And we declare with words of soberness, that an angel of God came down from heaven, and he brought and laid before our eyes, that we beheld and saw the plates, and the engravings thereon, and we know that it is by the grace of God the Father, and our Lord Jesus Christ, that we beheld and bear record that these things are true. And it is marvelous in our eyes. Nevertheless, the word of the Lord commanded us that we should bear record of it; wherefore, to be obedient unto the commandments of God, we bear testimony of these things. And we know that if we are faithful in Christ, we shall rid our garments of the blood of all men, and be found spotless before the judgment-seat of Christ, and shall dwell with him eternally in the heavens. And the honor be to the Father, and to the Son, and to the Holy Ghost, which is one God. Amen.
-Oliver Cowdery, David Whitmer, Martin Harris.
The Testimony of Eight WitnessesBe it known unto all nations, kindreds, tongues, and people, unto whom this work shall come: That Joseph Smith, Jun., the translator of this work, has shown unto us the plates of which hath been spoken, which have the appearance of gold; and as many of the leaves as the said Smith has translated we did handle with our hands; and we also saw the engravings thereon, all of which has the appearance of ancient work, and of curious workmanship. And this we bear record with words of soberness, that the said Smith has shown unto us, for we have seen and hefted, and know of a surety that the said Smith has got the plates of which we have spoken. And we give our names unto the world, to witness unto the world that which we have seen. And we lie not, God bearing witness of it.
-Christian Whitmer, Hiram Page, Joseph Smith, Sr., Jacob Whitmer, Peter Whitmer, Jr., John Whitmer, Hyrum Smith, Samuel H. Smith.
What makes you believe that all that the LDS church teaches is true?
That's a broader question, and kind of difficult to answer. Is there anything in specific, other than the word of Joseph Smith, that you would like to know why I believe in it?