Total equivalent resistance of simple series resistors is simply the numerical sum of all the individual resistors from one end of the power source through the circuit to the other end of the power source.

In left hand circuit, of the top picture, I show such a simple series resistor circuit. To get the numerical value of the total series resistance placed across the power supply, in this case from (+) to GND one simply adds up all the resistor ohmic values. This is a simple example, there are two resistors R1, and R2. If they were each 1000 ohms, the total would be 2000 ohms.

Knowing the total and the power supply voltage voltage, the current can be calculated. If the voltage were 10 volts, 10 divided by 2000 is .005 ampere or 5 milliampere, abbreviated as 5ma

In center circuit, of the top picture, the simple sum the resistance, and divide the voltage by the total is complicated by the fact that the bottom leg has two resistors in parallel. To make the calculation we now need some way of combining the two parallel resistors, to make them appear to be one, at least as far as our calculations are concerned. If R2, and R3 are each 1000 ohms, there total parallel resistance is 500 ohms. If you apply the formula in the bottom picture, for calculating you should arrive at this number.

Here's a Lab assignment, Acquire an inexpensive Volt/Ohm/Milliameter, often called a multi meter. This is not meant to be an endorsement, but Radio-Shack puts on sale about once or twice a year, for a smidgen less than $25.00 US a 43 range Multimeter. This is not a fancy electronic one, but rather is an electro-mechanical device, that has an analog, eg. pointer sweeping across a scale, as opposed to a digital meter. While you're there you should get a good assortment of resistors, whose values are in the range of 10 ohms to 10 meg (million) ohms. If you're planning to buy components for future elements of this course, consider an inexpensive power supply such as a wall wart, you probably have one of these laying in a drawer around the house. A 9 volt DC, one is preferred, but other voltages will work. A nice large low voltage Electrolytic Capacitor is useful, say... 10,000 uf (microfarad) rated to withstand 20 volts or more. Don't spend a lot on any of this stuff, most of can be obtained at a fraction of the cost at Electronic Surplus stores, or through Surplus Catalog outfits.

Once you have a multimeter, and some resistors in hand, read the owners manual for your meter, and select several resistors that are within a ten to one ratio, Highest value, to lowest value. I haven't taught you how to read the color bands yet, so use your meter to determine their ohmic value. Try wiring them in various series, parallel, and series-parallel circuits, try to predict the total resistance, using the formulas, and a pocket calculator, and see if they agree with your predicted resistance when you actually wire them up and make measurements with your multimeter.