Bookmark and download Flash from here, DO NOT trust any website telling you to install anything or follow their links.
Flash update
http://get.adobe.com/flashplayer/
Silverlight update
http://www.microsoft.com/silverlight/get-started/install/default.aspx
There is a FlashBack trojan making the rounds which tries to con people into installing it by looking like it's the real thing.
Make sure your Flash (and Silverlight) is current by running these checks
http://flashbuilder.eu/flash-player-version.html
https://www.mozilla.org/en-US/plugincheck/
You Mac likely will come across sites and content that won't play in Quicktime or in your web browser unless you install additonal codecs
You can get them here
Perian
http://perian.org/
VLC
http://www.videolan.org/vlc/download-macosx.html
Flip4Mac $29
http://www.telestream.net/flip4mac-wmv/overview.htm
How to install
When you click a link, a download of the installer file occurs to your Downloads Folder usually located next to your Trash Can or in your Users Folder.
Open the folder and double click (or File: Open) the download and icon can appear on your desktop which you open and inside is a installer package which you double click on to launch the installer.
OS X will check the download to make sure you intended to start it and that it's not malware it knows about.
Follow the installer instructions to install the software onto your computer and quit when finished.
Some require the browsers to be closed first, others require the machine to be restarted to enable the software.
Once the install is completed, remove any and all downloads from your Download folder, it's safer to keep this folder empty so you can keep track of what appears in there.
Drive by downloads of malware can occur and reside in the Downloads folder, mixing in with files residing there and await your click to install.
For backup, restore, performance and other tips I've got, read here
https://discussions.apple.com/thread/3358920