Kitin21, your question is rather broad, a robotic arm can be made from various materials such as aluminum, hard plastic or even wood. As to how to go about designing and building it, the skys the limit, well actually its limited by your mechanical skill level , the materials you have on-hand and the tools you have.
Lynixmotion sells robotic arms made from plastic, it uses DC servo motors and can be control from a computer with appropriate software.
Heres a link:
**broken link removed**