Help with Raspberry Pi and Arduino Motor Shield

So I have a project the we design to have a Raspberry Pi as the main processing unit and have an Arduino as a slave. For movement we are using a Adafruit Motor shield because we have problem with voltage drop with L298N Motor driver.
Since the Motor Shield uses I2C. How should we go about using the I2C for commands from the Pi?

Get the Raspberry Pi cookbook and read it. Also on line there are many videos etc showing the Rp and how to do what you want. This additional information will go a long way in getting your problem solved. Once you get past this you then need to select your sensors and actuators. At that point you start on the hardware design and write your software. Pick a different H-Bridge, the one you have will burn a lot of energy in heat. The Arduino cookbook will also help a lot. This will take some time to read this material but it will save you a lot more time when doing projects. That is how most of us learned.