I'd say the FOX is the car to learn on. A great blend of grip, speed, forgivingness and handling. If you can't drive that then there's not much hope for you, unless you're a major tintop lover. In which case the RB4/FXO and the front wheel drive GTR's are a good bet.
As for tracks, it's best to use an unknown car on a known track, so go for a few laps of Blackwood in the new car and see how different it is, and you'll quickly learn vague limits. Then go and attack Aston Historic or something - quite a long track, but it's got most 'types' of corner and as such teachs you a lot. Once you're happy with progress, try something else.
I find it best to always try on unknown at a time. If you try a new car on a new track it's a lot harder. If you know either the car or the track then you have an idea of corner speeds or grip levels etc.