What's the best ways to get into physics modding?

I'm not sure where to start, but I have started

 

Depends on where you are at now. If you know the actual physics of a car quite detailed, you have a good start I guess. Otherwise I'd start there.

If you know what kind of data you need, it's time to look for the actual values of the car you're designing.
Also take a look at the Dev apps in AC, as they give a nice overview of how some things are modeled.

Then take the physics SDK in the AC and look at the way all the values are entered. The SDK open physics data (all the non encrypted ini files, suspension, aero...) is based on the Formula Abarth car if I remember correctly.YYou can create a new car and play around with those values.

Not having created a full physics set for a car myself yet, I still see two main challenges, first is finding all the data. What you can't find (and for a normal car I guess that is the biggest part), you have to approximate or even guess. If you guesses a lot you need to compare the car to the actual one based on numbers and subjective feeling continuing to fiddle until it comes close.
Second challenge appears if you need to model something not available in AC, like rare suspension types, etc. But you can get a lot of help, partially here, otherwise for example at racedepartment.com.

Anyway, wishing you a good start and a lot of fun while tooling around with AC physics!