So you know the sum of the probability mass function has to equal one. This is the definition of a pmf. So set up the sum as Plato pointed out, set it equal to one, and solve for c.
i did some easier pmf problems in my text, where x goes from 1 to 4 and that's easy to solve for c.
but for this problem.... 1/2c + 1/8c +1/24c +....... = 1
but what value is it when it goes to infinity?