Calculating cell potential
In electrochemistry you always write the half reaction with reduced species in the left and oxidized species on the right. This is done for consistency purposes. Species with higher potential will reduce and species with lower potential will be oxidize. 

I$_2$ + 2e- ----> 2 I$^-$   +0.54     //will reduce (go from left to right), has positive charge
Cu$^{2+}$ + 2e- ---> Cu  +0.34       //will oxidize(go from right to left), has negative charge  

Next, you should use Nernst equation to each half reaction. You will get values E$_{Cu^{2+}/Cu}$ and E$_{I_2/I^{2+}}$. Now subtract one from another to get the voltage.