There are basically three different types of circuits that the resistor can be in.
Series
In this situation, to find the total resistance is to basically add up all the resistor values.
 R1 + R2 + R3 = total resistance " Rt " 
Parallel
To find the total resistance of a parallel circuit you would divide one by each of the resistors. Next add them up and once again divide one by your final answer. For example, if you have a 25ohm, 30ohm, and 18ohm set of resistors you would do the following. 1/25 + 1/30 + 1 /18 = .04 + .0333 + .0555 = 0.1288 1/0.1288 =7.76 ohms The total resistance of your series circuit is 7.76, notice that the sum of all the resistors are less then R1, R2, or R3.
1 + 1 + 1 1
R1 R2 R3 = Rt
Seriesparallel circuit
In the following example we have a circuit that is a combination of both series and parallel circuits. The equation for solving this circuit is to first find the values of your parallel circuit. The next step is to treat the total resistive value of your parallel circuit as one resistor. With that in mind you will add the total resistance in your parallel circuit with the remaining series resistors to get your final value.
 1 + 1 = 1 R2 R3 Rt + R1 + R4 

 