KDE is a desktop environment, not an entire operating system. KDE is somewhat like what Windows was back in the days when Windows ran on top of DOS. DOS was the Disk Operating System, and Windows was an environment that ran atop it.

Debian is a Linux distribution. Meaning that it is Linux--a small piece of an operating system, not useful by itself--plus a selection of other odds and ends that fit together to provide an operating system and a wide selection of applications that run on that operating system.

KDE can be installed and run on Debian.

So what can i do?

I'd suggest that, if you're really interested, you should get a beginner's book for Linux.
On the system level, Linux and Windows are mostly using the same concepts. The difference with Windows is that the low-level stuff is all hidden behind a graphical user interface, and often not accessible or only with great difficulty. With Linux the low-level stuff is accessible by the user (which is a good thing if you want to fix a problem yourself). This can often confuse new users, but it's not needed to get started.
The internet tablet, of course, is a consumer device and so Nokia put a user interface on top to be easily usable by everyone. Normal users would never see the Linux beneath. But if you want to leave the path of being a consumer who just uses the tablet for what it was advertised for, you can do so, because it runs Linux. The door to the powerful low-level layers of the tablet is the X-Terminal in the Utilities menu.
At a first glance, the X-Terminal appears to look and work a bit like DOS or cmd.exe in Windows. The command line is very powerful, if you're willing to dig into it, and the basic stuff is not very difficult. As a side-result you will also get to understand the basic principles of Unix, which can also be helpful when working with MacOS-X or Solaris or other Unix-style systems.

But we warned: Learning Unix takes some time.
But it never hurts to know the basic shell commands like 'ls', 'cd', 'rm', 'cp', 'mv', 'ln', 'ps', 'kill'. These few commands can already get you quite far.

I agree, Maemo is too closed. I would scratch the OS2008 Maemo Diablo in a second if KDE runs on it smoothly. Message to Maemo: open your system more!

For graphics there are a few tools with which you can do some simple sketching. Liqbase, for example. This can be nice to sketch ideas/inspiration (I don't know what kind of graphics work you do but it might be an aid).

You can make a network with your PC using the USB wire, using BlueTooth (USB BlueTooth stick costs 5 EUR), or using WiFi (ad-hoc or by setting the PC into master/ap mode).

Hi guys,
above all thanu everybody for the answers youre givin me!
Then...

I saw on kde.apps.org some apps like:

Pixel Image editor;
Kad,
etc.

Can I use them on maemo?
Is there a way to make them available on maemo?

Wacom makes touch screens for graphics designers.

The NIT isn't made for this, and AFAIK there is no such port for Maemo so this won't work.

For KDE there are some graphics manipulation programs: Krita is KDE's Adobe Photoshop alternative and Karbon14 is KDE's Adobe Illustrator alternative. They work very well, especially the 2.0 tree is advanced feature-wise (but still in beta). They are part of KOffice. I don't know if KOffice works on Deblet though.