Is there a way to be sure I will become a Field MP rather than a Garrison MP?

Everybody must understand, once you are in the military you have very little control over your life. You can put in preferences on what you'd like to do, or where you'd like to be stationed, but it's a lottery. All Army wants to go to Germany instead of Korea, or Iraq. So what's your chances of getting a getting Field MP? Might happen, might not.