I'll let others address the Credentials, but for starters, you'll want to run a few High Performance Driver's Education (HPDE's for short) Open Track Days.
Instruction is always good, but is not necessary to simply try out an open track day to wet your feet the first time.
Just don't drive any harder than you are comfortable with.
AFAIK, NASA will then REQUIRE that you graduate to some series of actual INSTRUCTED HPDE's, so that you may be "Certified". (To what level, I'll leave to others who know to explain).
Seat Time, Seat Time, Seat Time.
Getting into Auto-X every chance you get is a GREAT tool too.
Auto-X is another beast due to the speeds, but many of the lessons learned translate well.
After you get into Track driving, you may or may not continue too much w/ Auto-X, due to various factors, but it never hurts to get some Auto-X under your belt.
MANY (Not All by any means) of the BEST drivers have a few years of Auto-X under their belt, even if it was years ago.
Top running Trans-Am driver Johnny Miller is from my Region, and he even came back to run an event in his street 'Vette a couple years ago just for fun.
Auto-X is where he started, and he's still QUITE an Ace.
