The choice to go on a mission is completely voluntary. Of course young males are "urged" by their church leaders to go. It's almost kind of like a right of passage. That being said however, a significant number of young males choose not to go for various reasons and their standing in the church and community is not affected. Steve Young, a very prominent Mormon, did not go on a mission and instead chose to play college football all 4 years and then play in the NFL.
If one does want to go on a mission they must go through an application process and be interviewed by their local church leader. They put a "starting date" on the application indicating when they are available to start (for example "June 2009"). Church headquaters will then send them a letter indicating where they will be going and what date they will be starting. Missionaries do not receive any financial compensation for the work and actually pay their own way. Again, the potential missionary intiates the process and notifys when he is available.
Bo McNally is a saftey for Stanford and I actually played against him in High School - Timpview won

. He played all four years at Stanford and did not go on a mission. He is a great kid and a strong member in the church. I don't know what is situation is now but he could be gonig on a mission after he graduates or he might not.