[nathan_hui]

So an encoder is simply a sensor that tells you how much the encoder has rotated. For moving an arm to a specific position, you need to use a control algorithm such as PID. PID works by calculating a power setting for the motor based on the arm's current position (taken from the encoder) relative to the position you want the arm at. If you're in FTC, I believe you'll be using either NXT-G or RobotC (are there others?), however, I'm only familiar with RobotC.