I would read through this thread:
http://www.fightingrobots.co.uk/thre...ght=Pneumatics
It has tonnes of information and questions already asked and answered about pneumatics. Also search for similar threads and read them.
You should especially read through this website:
http://www.teamdavinci.com/understanding_pneumatics.htm
It is absolutely brilliant and explains everything you need to know.

Once you've done some research then we can answer more specific questions.