How can I connect arducopter to Raspberry Pi?

To do so, you need to use a USB link between the Ardupilot/Arducopter and use the MAVLink protocol to interrogate the Arducopter in order that it sends back its GPS data. You can etheir way use the C libs of MAVLink, the Python implementation or the ROS one (named mavros). You may find some clues here:

• MAVLink dev guide
• Python implementation - the easier one to handle

Depending on your programming skills this may requires you some time to reach your aim!