go ask a recruiter. seriously this **** does not belong here.
You can rarely choose a specific job. You usually just choose you're field like I did (3xx infantry). Infantry is hard to come by now a days because the Corps is downsizing and everyone wants infantry, but good luck in getting it!
You can only pick your exact job if you enlist into the reserves. If you go active duty you can only pick your job field of 03xx.
While in infantry school, you may or may not be allowed to pick your exact job. Many Marines who want 0311 do get it since it is the largest job in the infantry. Just understand you can be placed into any of the infantry jobs.