Use an L293D with Arduino
All the inputs are clearly marked.

The battery connects to +12V and GND
The Arduino's 5V and GND connect to 5V and GND.

The control signals connect to the header of control signals. ENx is the PWM signal for the speed for motor X. IN1/IN2 are the direction/brake controls for motor 1, and IN3/IN4 are the direction/brake controls for motor 2.

The signal labels are exactly the same as the pin names for the chip and match up precisely with every single tutorial on the net for using the L293D with the Arduino.