You connect a RPi to Arduino using USB, just like to any other PC/computer.
The 256 MB version of the RPi only has one USB port. If you want to use USB to connect both an Arduino and a Webcam, that won't work (without a separate hub, which increases price and weight.)
If you go with the Raspberry Pi specific camera, rather than a USB webcam, then you can use the remaining USB port for talking to the Arduino.
If you want to get started with Raspberry Pi programming, there's a good forum over on raspberrypi.org. They're quite friendly to newbies.
Also, there exists a distribution of ROS for Raspberry Pi. This will require more learning up-front to get going, but will also then give you a lot of pre-built functionality, like vision tracking, navigation control, etc. You'd probably need the 512MB version to run ROS.