It's an over-generalization but a car typically gets more expensive as the classes go up in performance - ITC, ITB, ITA, and ITS, from least to most expensive (assuming similar levels of preparation).
ITC seems typically to have the smallest classes but depending on where you race, any of the classes offer someone to race with.
Your decision might hinge on what kind of car you like, as some makes are competitive (or even eligible) for some classes but not others. Otherwise, your decision might come down to what you can find that is already well prepared. Have fun!
Kirk