You must be an SCCA member to participate in the SCCA solo events. If you are not a full member, you can buy a weekend membership. There is no charge to spectate. You need to sign the waiver, then you will be able to do ride alongs.
There are 4 autocross clubs in the DFW area. All of them have loaner helmets. Buying your own is a good move, also, waiting for the Snell 2015 is another good move.
There is an autocross this Saturday, the 19th, at the TMS bus lot. http://www.dfwmiata.com/threads/2736...15-TMS-Bus-Lot
jrj512 also maintains a calendar at http://www.dfwautocross.com/