ain, I agree with and recommend all the comments above 100%.
That being said, the first step would be to look around the web for parts. Find some pneumatic actuators, then an air compressor. Those components alone will probably cost you about ~$300 (I'm not sure what your budget is). Note that the ultra cheap small air compressors are known to be annoyingly loud.
Also, draw up some detailed designs that include every screw, etc.
After you complete those steps, report your progress here and we can offer more advice.