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Mormons hold that there is no real Trinity, no original sin, that Christ did not institute baptism

We believe in the Godhead: God, the Father; His Son Jesus Christ, and the Holy Ghost.

And no, we don't believe in Original sin. Because of the Fall of Adam and Eve, all people live in a fallen condition, separated from God and subject to physical death. However, we are not condemned by what many call the "original sin." In other words, we are not accountable for Adam's transgression in the Garden of Eden. See the 2nd Article of Faith

2 We believe that men will be punished for their own sins, and not for Adam’s transgression.

If you want to read further on our beliefs about the Fall of Adam, go to Fall of Adam and Original Sin

We do believe that Christ instituted baptism. From the New Testament

Acts 19 3-5
3 And he said unto them, Unto what then were ye baptized? And they said, Unto John’s baptism.
4 Then said Paul, John verily baptized with the baptism of repentance, saying unto the people, that they should believe on him which should come after him, that is, on Christ Jesus.
5 When they heard this, they were baptized in the name of the Lord Jesus.

Here is the definition of baptism from lds.org

Baptism by immersion in water by one having authority is the first saving ordinance of the gospel and is necessary for an individual to become a member of The Church of Jesus Christ of Latter-day Saints and to receive eternal salvation. All who seek eternal life must follow the example of the Savior by being baptized and receiving the gift of the Holy Ghost. From Acts 2:27-28
37 ¶ Now when they heard this, they were pricked in their heart, and said unto Peter and to the rest of the apostles, Men and brethren, what shall we do?
38 Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost.
From the Book of Mormon 2 Nephi 31:3-13:
4 Wherefore, I would that ye should remember that I have spoken unto you concerning that prophet which the Lord showed unto me, that should baptize the Lamb of God, which should take away the sins of the world.
5 And now, if the Lamb of God, he being holy, should have need to be baptized by water, to fulfil all righteousness, O then, how much more need have we, being unholy, to be baptized, yea, even by water!
6 And now, I would ask of you, my beloved brethren, wherein the Lamb of God did fulfil all righteousness in being baptized by water?
7 Know ye not that he was holy? But notwithstanding he being holy, he showeth unto the children of men that, according to the flesh he humbleth himself before the Father, and witnesseth unto the Father that he would be obedient unto him in keeping his commandments.
8 Wherefore, after he was baptized with water the Holy Ghost descended upon him in the form of a dove.
9 And again, it showeth unto the children of men the straitness of the path, and the narrowness of the gate, by which they should enter, he having set the example before them.
10 And he said unto the children of men: Follow thou me. Wherefore, my beloved brethren, can we follow Jesus save we shall be willing to keep the commandments of the Father?
11 And the Father said: Repent ye, repent ye, and be baptized in the name of my Beloved Son.
12 And also, the voice of the Son came unto me, saying: He that is baptized in my name, to him will the Father give the Holy Ghost, like unto me; wherefore, follow me, and do the things which ye have seen me do.
13 Wherefore, my beloved brethren, I know that if ye shall follow the Son, with full purpose of heart, acting no hypocrisy and no deception before God, but with real intent, repenting of your sins, witnessing unto the Father that ye are willing to take upon you the name of Christ, by baptism—yea, by following your Lord and your Savior down into the water, according to his word, behold, then shall ye receive the Holy Ghost; yea, then cometh the baptism of fire and of the Holy Ghost; and then can ye speak with the tongue of angels, and shout praises unto the Holy One of Israel.
And from Alma 7:14-16:
14 Now I say unto you that ye must repent, and be born again; for the Spirit saith if ye are not born again ye cannot inherit the kingdom of heaven; therefore come and be baptized unto repentance, that ye may be washed from your sins, that ye may have faith on the Lamb of God, who taketh away the sins of the world, who is mighty to save and to cleanse from all unrighteousness.
15 Yea, I say unto you come and fear not, and lay aside every sin, which easily doth beset you, which doth bind you down to destruction, yea, come and go forth, and show unto your God that ye are willing to repent of your sins and enter into a covenant with him to keep his commandments, and witness it unto him this day by going into the waters of baptism.
16 And whosoever doeth this, and keepeth the commandments of God from thenceforth, the same will remember that I say unto him, yea, he will remember that I have said unto him, he shall have eternal life, according to the testimony of the Holy Spirit, which testifieth in me.
From 3 Nephi 11:18-41 (I would recommend reading the whole chapter, I won't post it here)
23 Verily I say unto you, that whoso repenteth of his sins through your words, and desireth to be baptized in my name, on this wise shall ye baptize them—Behold, ye shall go down and stand in the water, and in my name shall ye baptize them.
Some more references about the Church of Jesus Christ of Latter-Day Saints and our beliefs that Jesus Christ did institute baptism:
3 Nephi 27:13-22

Doctrine & Covenants 75:50-53