I used to develop desktop applications for Windows, I know how to develop using C++, C#, and Java. I would like to start development for Ubuntu, how to get started?
The best place to get started is the new Ubuntu Developer Portal: http://developer.ubuntu.com
You can use a variety of IDEs (some may even be familiar), such as CodeBlocks, Netbeans and Eclipse. It will all boil down to choice, if you ask me. Installing an IDE is pretty easy too: Just open the Software Center and search for the one you want or browse the development category.
If you already know how to code in the languages you mentioned, then the main difference you will find on Ubuntu will be in compilation. Even so, it's easy to adjust, especially when letting the IDE do most of the "dirty work" for you.
Ubuntu does not have a special framework like Window's .NET or Win32 API. The somewhat special to Linux is the POSIX enviroment, this provides IPC, shared memory, threads and such. Or did you mean GUI? Gnome is written using GTK+ und KDE using Qt. Using those gives you a native look, if you mean that.
Can you specify a little more what exactly you want to know?