Write down all of the information that you are given in the problem. If a teacher expects you to calculate the resistance of a circuit component, she should give you the voltage through the circuit, usually measured in volts (V) or millivolts (mV). You should also be given the current through the circuit, which is most often given in amperes (A) or milliamps (mA).
Convert all of the units in your problem to volts and amperes. If you do not convert your units properly, you will probably end up with an incorrect calculated resistance. In order to convert millivolts to volts or milliamps to amps, divide by 1,000. For example, if the current in your circuit is 20 mA, divide 20 by 1,000 to get 0.2 amperes.
Divide your voltage by your current to calculate the resistance of your circuit. For example, in a regular wall circuit that carries 120 volts, you would divide your current (0.2 A) by 120 to get 0.017 Ohms as your final resistance.
Make sure that your answer is reasonable. If you get an answer that is unreasonably large or unreasonably small, go back and check your work.