Step 1: What You Will Need.
1.I used a 2n2222a NPN transistor but most other NPN transistors that are similar to this will work if you have any questions ask in the comments.
2.One 1K resistor(brown, black, red)
3.One 10K resistor(brown, black, orange)
5.A small jumper wire.
Step 2: Putting in the Transistor
Place the transistor in the breadboard so each pin is in a different row.
Step 3: Adding the 1K Resistor.
Put the resistor between positive power on the breadboard and the collector on the transistor. This resistor limits the power to a safe amount for the LED.
Step 4: Adding the Light.
put the LED between the emitter of the transistor and ground. The light will indicate when the transistor is activated.
Step 5: Adding the Button.
Place the button in the center of your breadboard close to the transistor. The button will activate the transistor.
Step 6: Adding the 10K Resistor.
The resistor goes from one side of the button to positive. the resistor limits the power to show a small amount of power can control a larger amount with a transistor.
Step 7: Connecting the Button to the Transistor.
Add the small jumper between the output of the button and the base of the transistor. When current is applied to the base it triggers the transistor and allows current to flow from the collector to the emitter.
Step 8: What Can This Be Applied To?
You can use a transistor to control a larger amount of power than an IC, microprocessor(like Aurduino), or some other circuit can handle. Like if you wrote a program in Aurduino for lights to progressively get lighter as the ambient light level gets darker the Aurduino could not provide enough power for the lights and a relay cant adjust the amount of power it outputs so you would have to use a transistor.