You'll need to me get an AMA Card (http://www.americanmotorcyclist.com/For-Members/Join-The-AMA)
Practice, then go race. In D6 for 2017 they have the Quad Ultimate Series (http://www.d6mx.org/series.html), had 8 races.
Try the QUAD Reps listed on this site for more info:
Quad REPS: Michelle Stillo & Rob Boehner 973-970-4422