My recommendation, in order to ensure you can install the correct and necessary applications, that you first setup your Software Package Management repositories. Specifically I recommend you setup your software repositories (where repositories are in essence file servers on the internet with software for openSUSE) with the repositories OSS, non-OSS, Update, and Packman. Just those 4. No others. None! There is guidance here for that for openSUSE-11.1:
Repositories/11.1 - openSUSE-Community
You can add more than those 4 only after you understand the risks, and understand how to identify and fix problems that can arise if you add more than those 4 repositories. This is beyond the capability of new users, and indeed beyond many average users. So it may be best for you to just stick with the 4 I recommended.
Once those 4 are setup, then go to YaST > Software > Software Management and change the filter to "search". From there you can search for and install applications that are on those 4 repositories (with your PC connected to the Internet).
For Firefox I recommend flash-player and Mplayer and mplayerplug-in be installed. For general multimedia I recommend you replace the Novell/SuSE-GmbH crippled versions with Packman packaged versions. Specifically libxine1, amarok, amarok-xine, amarok-packman, vlc, smplayer, libffmpeg0, w32codec-all, libquicktime0, libxvidcore4. You could also install the Packman gstreamer versions (replacing the Novell/SuSE-GmbH packaged versions). In most multimedia applications you can select the appropriate sound engine (I recommend xine engine).
That should get you started.
I do NOT use banshee, so I can not help you there.