The Church of Jesus Christ of Latter-Day Saints teaches that Baptism into the Church of
Christ is necessary to enter the
Gates of Heaven. See
Baptism for the Dead for an explanation of what happens to the rest of the world. The Church teaches that it is the only Church with authority to baptize, somewhat due to
The Apostasy. Mormons believe that baptism is done for
the remission of sins and to officially join the Church. Baptism is considered a
covenant in which the person pledges to follow the commandments. Mormons believe that Baptism must be done by
immersion in water. It can be argued that Christ was baptized like this. It also goes along with the idea of baptism symbolizing being buried and
born again.
To be baptised into the Church of Jesus Christ of Latter-Day Saints you need to be at least eight years old. This is considered the age of accountability. An interview with either a Bishop or missionary usually precedes baptism to make sure the person understands the basic principles of the Church. Baptism can actually be a disservice to someone who does not intend to follow the commandments as it increases your responsibility to do so. Therefore, a person cannot be baptized unless they agree to refrain from sex outside of marriage, follow the Word of Wisdom, pay tithing and, in general, follow the commandments.
Baptisms are usually done at a Church building, all of which have baptismal fonts. Any body of water will do fine though, and rivers/lakes are occasionally used. The person performing the baptism and the person being baptized wear all white clothing. Both enter the water and the Baptizer raise his right arm and says, "Having been comissioned of Jesus Christ, I baptize you in the name of the Father, and of the Son, and of the Holy Ghost. Amen." The person is then lowered, usually backward bending their knees, into the water. The person must go completely under the water. Often when young children are baptized a toe will stick out and the baptism starts over with the prayer. This is done because Satan only needs the tiniest bit of flesh to drag you down to Hell*.
After the Baptism the person will usually go dry off and put on some dry clothes. Originally they were then confirmed, but recently this has been changed to take place during the next Church service. Confirmation involves someone holding the Priesthood placing their hands upon the head of the person and conferring upon them the power of the Holy Ghost. Usually during the baptismal service there are a few songs, a few short talks and a few prayers.
* Ok, this isn't true, but it's fun to think about.