Write down all the information that the problem gives you. To calculate the intracellular potential using the Nernst equation, you will need to be given the concentrations of the relevant ion.
For example, a typical Nernst equation problem may ask you to calculate the intracellular potential for sodium, given that the intracellular and extracellular concentrations of sodium are 30 millimolar and 150 millimolar, respectively. Remember, the concentration of the ion is not the same as the electrical potential due to that ion.
Divide the extracellular sodium concentration by the intracellular sodium concentration. In the above example, you would divide 150 by 30 to get 5. Make sure that you do not switch the two; if you do, you will calculate the extracellular potential instead of the intracellular potential.
Take the log of your answer in Step 2. You will need to use a scientific calculator for this. Make sure that you take the base 10 log, usually abbreviated "log" on the calculator, instead of the natural log, which is often abbreviated as "ln." In this case, the log of 5 is 0.698. Remember to keep more decimal places than you need to avoid rounding errors later.
Divide 58 by the charge of the ion for which you are calculating the potential. The problem may give you the charge, or you may have to find it from a periodic table. Since the most typical charge for sodium in a biological cellular system is +1, you would just divide 58 by 1 to get 58.
Multiply your answer from Step 3 by your answer in Step 4. Here, you would multiply 58 by 0.698 to get 40.48.
Make sure that your units line up correctly. For example, the potential here is 40.48 millivolts because the problem supplied concentrations in millimolar amounts. However, if the problem gave you concentrations in molar amounts instead, you would report your answer in volts instead of millivolts.