Welcome.. and I am sure someone will sign on and direct you to the correct class.. if you happen to register for an event and select the wrong class, that can always be changed. I am not sure where the Fiesta ST goes my guess would be HS- as long as the width of your wheels themselves were stock.
As far as events- just sign up! One thing to remember, bring a cooler with lots of water, and usually a lunch, you’ll need a sun hat and sunscreen..you’ll run a heat and work a heat, so that can mean standing on the course shagging cones in the heat. There will usually be a novice walk of the course before the event, and I suggest walking it more than once.. there are also people who can hop into your car and help you- it can appear as a sea of cones when you start. But come out and have fun, and you will soon be addicted to autocross. Everyone is pretty helpful.