I been in the service for 3 years now I am National Guard but I was wanting to go SF but there is a catch I?

was arrested at 18 for a possession charge which really wasn't mine had no clue that it was in the car. My mistake as a kid would that hinder me from go SF. I heard I could hire a lawyer and get it expunged from my record would that even help? Any information would help I really would like to be a part of the most elite soldiers that walk this earth and wear that GREEN BERET!!!