from where should i download?

For Yahoo! Messenger, you may want to use Empathy (click the menu with your username near the upper-right corner of the screen and click Chat Accounts...) or Pidgin (you can install it in Applications > Ubuntu Software Center). These programs support multiple chat protocols, including Y!M, and they tend to work extremely well.

However, if you want to try to use the Yahoo! Messenger application from Yahoo!, there's a version for GNU/Linux systems that might work, though it's rather old. Download http://in.dld.yahoo.com/i/in/messenger/linux/debian_sid/ymessenger_0.99.17-1_i386.deb. Double-click on that file to install. If it doesn't put an entry in your menus, you should be able to run it by pressing Alt+F2, typing "ymessenger" (without the quotes), and pressing enter. If it doesn't put an entry in your menus and you want help manually adding it, please post again.

To install Skype, go to Applications > Ubuntu Software Center. Go to Edit > Software Sources..., click the Other Software tab, and check the box for Canonical Partners. Click Close. At the bottom of the Ubuntu Software Screen in the left pane, under "History", you may see an item called "In Progress". Wait for that to disappear. Now, in the search box at the upper-right corner of the window, type in "skype" (without the quotes). When Skype comes up as a search result, click on it, and click the Install button.

Unfortunately there is no version of Camfrog for Ubuntu. However, you can probably run one of the older (5-series) versions of Camfrog for Windows, using Wine (http://winehq.org). Wine makes it possible to run some Windows programs on Ubuntu. You should be aware that many programs, including Camfrog, are likely not to run as well with Wine on Ubuntu as they do normally on Windows. See http://appdb.winehq.org/objectManager.php?sClass=application&iId=3710 for details about what versions of Camfrog are known to work with Wine. To install Wine in Ubuntu, type "wine" in the search box in the Ubuntu Software Center. The search result "Wine Microsoft Windows Compatibility Layer" will come up--click on it, and click Install. Once Wine is installed, you can right-click on the Windows installer for Camfrog, and click Open With Wine.

If you need further help with any of this, please post again.