I have 12/2 wiring coming into the heater which is 240 from a 2 pole 15 amp breaker... so 3 wires a white, black and ground... inside the heater is a black and white from the thermostat and 2 blacks to the heater core , now how do I connect all these ?

What is the brand and model number of the heater?

Most likely it will be:
Line in white to one heater wire
Line in black to one t-stat wire
The other t-stat wire to the other heater wire.

I am concerned because I don't remember ever seeing a t-stat with black and white wires.
Also, it may be factory wired somehow.

Okay , sorry it is two black wires coming out of t-stat and a b & w coming out of heater core.. so now how does it then get connected to 3 wires coming in being 12-2 wire

Well, without seeing the instructions this is an educated guess:

White from wall to white from heater.
Black from wall to black from t-stat.
Other black from t-stat to black from heater.

Got it working thanks stan