Here is a good introductory video tutorial to Ubuntu. That youtube user has dozens of other tutorial videos as well. He's using an older version of Ubuntu, but most of the basics are the same, assuming you aren't using the newest version (11.04), which releases in a couple days (April 28, 2011). Version 11.04 uses the new Unity layout, which is pretty radically different, and much more akin to OS X.
Here is the official documentation for Ubuntu. It's much more in-depth (and maybe a little more work to wade through). It will walk you through most of the basics.
In short, to get to places, you'll want to use the three menus in the upper-left of the screen, especially the "Applications" menu. Under Applications > Internet you'll find firefox, which is the default web browser. The Open Office programs will do the work you may be used to doing in MS Office (word processing, spreadsheets, etc.). The last item in the Applications menu is the installer, where you can search for new programs and install them.
If you have a more specific problem than this, go ahead and edit your question (or ask a new one) and we'll be able to help further.