This is the simplest circuit you can get with one transistor. Any NPN transistor can be used.
Connect the LED, 220 ohm resistor and transistor as shown in the diagram.
Touch the top point with two fingers of one hand and the lower point with fingers of the other hand and squeeze.The LED will turn on brighter when you squeeze harder.
Your body has resistance and when a voltage is present, current will flow though your body (fingers). The transistor is amplifying the current through your fingers about 200 times and this is enough to illuminate the LED.