I'd do a lot of public racing before seriously considering entering a league/championship. There's a lot you have to be familiar with to do well in a championship, some of which can only be gained with experience. Hell, I've had LFS for 9-10 months and I still don't consider myself ready for such a thing, even in the FOX which I'm most comfortable with.
The FOX is popular for public racing though so it usually isn't difficult to find a good server.