Q,
David O. Mckay, former prophet of the church, said, "Every member a missionary."
http://www.lightplanet.com/mormons/conferences/97_oct/scott_why.htm
This was the "Call to Arms" for all to serve spread the gospel particulary young men. Mormons believe this is talked about in New Test., Daniel 2:44-45
44And in the days of these kings shall the God of heaven set up a kingdom, which shall never be destroyed: and the kingdom shall not be left to other people, but it shall break in pieces and consume all these kingdoms, and it shall stand for ever.
45Forasmuch as thou sawest that the stone was cut out of the mountain without hands, and that it brake in pieces the iron, the brass, the clay, the silver, and the gold; the great God hath made known to the king what shall come to pass hereafter: and the dream is certain, and the interpretation thereof sure.
Missionary work is the "stone" the will cover earth.
To be a missionary you have to follow the Word of Wisdom (no alcohol/smoking/coffee) have faith the church is true, not have had sex in the past year (personal gratification is frowned upon) and be a law abiding citizen, etc.
If the person feels they fit the requirements they will have a meeting w/ the bishop, then the stake pres. just to make sure you are ready. The stake pres. will also ask you to tell him ALL your sexual sins, regardless if you have repented of them.
Once that's done, paperwork is filled out and sent to church headquarters located in Salt Lake City, Utah. The church claims every missionary is sent where God has inspired them to be sent, but more than likely they send the kid wherever an extra body is needed.
If a new language is to learned the kid goes to Missionary Training Center (there are a handful throughout the word) where they are surrounded by other missionaries and all day long are bombarded with the language and the proper customs and how to teach the lessons.
Recently Hinckley has relaxed the Every member a missionary thing because a high amount of pressure was being placed on these young kids. He preaches that only those who fully feel prepared should go.
It is common in the city where I grew up that all young men went on missions. Quite a few went becuase the parents would buy brand new cars for them on ther return.
Such is life.