How can I calculate galvanic cell potential?
You calculate galvanic cell potential by subtracting the electrode potentials of each half-cell.
An electrochemical cell (I assume this is what you mean by a galvanic cell) consists of two half - cells. If you know the electrode potentials of each half - cell then you can work out the potential difference
For example suppose we set up a cell using the two half - reactions:
To get the overall cell reaction you can tell from the electrode potentials that the zinc half - cell will form the negative side of the cell i.e electrons will flow from the zinc half - cell to the copper half - cell.