If this is a fresh installation of Ubuntu then you need to install the ubuntu-restricted-extras package, not just for your flash player but also many of the other plugins & codecs your going to want along the way.
You can install all the plugins & codecs separately of course but the restricted extras gets most of the stuff your likely to need in one go.
You`ll find it either in Synaptic via your System>Admin menu or you can install it via the terminal if you`re comfy enough with it.
Code:
sudo apt-get install ubuntu-restricted-extras
EDIT: And for future reference...
http://www.ubuntupocketguide.com/index_main.html