Control 12V DC via MOSFET and normal transistor
The s8050 allows the 5V Arduino output to control the MOSFET gate which requires at least 10V. It also inverts, so high= off.  There should be a base resistor, 1K or 470 ohms would be fine. Any general purpose NPN silicon transistor will work here. 

The turn-on is rather slow because of the relatively high 10K pull-up, so this should be used with a slow PWM frequency to minimize heating of the MOSFET.