Earn your commission through a commissioning source.
For USMA or ROTC, you must get selected based upon your rank on the OML
For OCS, you must have a Top Secret clearance submited prior to submitting packet for branch within OCS class. Then you must select MI if there are slots when it comes your turn to select.
From your commissioning source, you would go to MI BOLC to learn how to be an MI Officer.
OML=Order of Merit list. Sorry...I was exhausted when I typed this, so I did not spell it out as much as normal.
The OML is a ranking of every commissionee within that commissioning source for that year.
USMA will have it's own OML and ROTC will have it's own. I honestly can NOT speak for USMA's OML system. ROTC, it is based on GPA, PT, peformance in the program, and other things such as leadership in the community, etc. The top 10% of ROTC commissionees will get their first choice of branch guaranteed.
Here is one little tricky thing about Military Intel for a branch....many of those selected for MI are branch detailed first. Since MI has a larger need for Captains and above and less of a need for Lieutenants, this is very common. So if you are selected for MI, but are branch detailed you will go to another branch's BOLC instead of BOLC for MI and serve in that branch for approximately 3-4 years before transitioning to MI. I know several who are doing this now. Infantry, Armor, Field Arillery and Chemical are common branches for 2LT's to get detailed into. In my mind, this works out better for the LT that gets detailed than it does for those who go straight to MI. Those who are detailed will get a vast amount of operational experience that will translate well into the MI world.