Using the classical definition of probability, it is:
probability of an event =
# ways the event of interest can occur / # possible events
...and this is a situation of conditional probability, since we are given that it is a head.
The probability that the coin is a two-headed coin is 0.5, since there are two coins that can produce a head (we are given the information that it is a head), but only 1 of the 2 is a two-headed coin.
P(2-headed | head) = 1/2