How do I connect a samsung galaxy mega via bluetooth to a jvc car stereo?

Set one or the other into search mode, usually set the radio into broadcast, set the Samsung into search to look for the broadcasting signal, once the SS finds the radio usually a command sequence initiates the paring sequence, that varies upon devices.

Then it gets paired. Then Voila, you have them paired.
Then you should be able to play and such through your radio.

Note look in your manual for the broadcasting ability on your radio, it will vary from model to model different buttons and such.