Best of luck in your project,
Maria E. Cipollone
Without specific info you get answers as varied as concrete panels to bamboo (neither of which I would consider appropriate to contain or protect a garden.
Mark LEach | http://www.americansecuredfence.com/RESIDENTIAL-COMMERCIAL-I
Zach | http://www.alpinefenceglenview.com
Michael Foster http://www.qualitychainlinkfencing.ca/services.html
Wood is nice, but also suseptible to insects, particularly termites.
Metal (wrought iron) can look really nice as well, but does require some maintenance from time to time. It can provide a "richer" look however when done well.
As the other poster mentioned, if you live in an HOA, make sure you consult your CC&Rs before doing anything.
Good luck! Home improvements can be a lot of fun.