call a gun store nearby for requirements or visit their website. no one's going to get suspicious if you actually ask them unless your voice sounds like a child's.
be 21
have state id
background check before purchase (criminal, mental health history). If 5150'd, purchasing will not be possible for the next 5 years. this can be challenged in court.
complete firearms safety course or on-paper test
proceed with application process (finger print, fees)
other requirements depending on the state
some store owners/employees will be judging a new purchaser based on how they look and behave for obvious reasons. even if qualified, they can choose not to sell.