I am installing a 240V baseboard heater. This was prewired into the house but does not have a breaker as yet. The pre-installed wire is 12-2 with ground. My question is what kind of breaker should I purchase. I think I would need a 20 Amp Douple pole, am I right?

Yes that is correct, be sure the breaker is a 2 pole 20 amp rated for 240 volt with a handle to tie both poles.

There are breakers that look like 2 pole but have 2 separate handles and are really for two 120 volt circuits.