There are many ways of connecting the Raspberry Pi and Arduino, such as using the GPIO and Serial pins and using I2C. But this could be one of the easiest way to get them talking, because hardware that required is minimal: all you will need is a micro USB cable that comes with the Arduino.
To Demonstrate how this works,here are two little projects, one for data going to Raspberry Pi from Arduino, the other one for the opposite. First of all, make sure you have installed pySerial, which gives you the ability to read from and write to the serial port with Python Programming language.
1.Arduino Talking to Raspberry Pi via USB cable
2.Raspberry Pi Sending Data To Arduino