Well you haven't done a race yet, so I suggest you do some Legends and MX-5 races (especially with Okayama coming out very soon) to try out both oval and road, and then once you get your SR to Rookie 3.0 you can use the Street Stock, Solstice and also the Spec Racer Ford all without spending any money.
You should also check out the series overview for road and oval in the reference section, but for road the Skippy is a must tbh as the most used non-rookie car, and also the fact it's good trainer car. After that it's a case of whatever you want to do.