An AT-6 (aka as the Texan or SNJ for the Navy) is a really good plane to start with. It flies like a pattern plane so will allow the leearning curve with warbirds to be smoother. Learning the set up of retrats and flaps that most other planes don't use and then going to more advanced types like the P-51 or P-47 would be good second choices. The German Stuka is a good first plane and has the advantage of fixed gear.

Planes to avoid as a first attempt:
Twins (P-38 etc)
World war one particularly the bipes and tripes with short noses.

Good luck and happy flying.
EXCAP232