What tools or software or devices are neccessary to create proffesional drawings for games for example? How to draw something and be able to use it on PC?
If you don't have some kind of pen tablet, then that's the first thing you need to get if you want to do any kind of digital drawing/painting/illustration. You can use an iPad, Android tablet, Wacom tablet, tablet PC (e.g. a laptop with a swivel tablet screen), or possibly even a smartphone with a touch screen (e.g. via remotedroid or premotedroid).
Besides that, any professional graphics program with drawing/illustration capabilities will work:
If you go on DeviantArt, a lot of the professional illustrators and artists on there include the materials they used for each piece, so you can use those as a reference if you want.
Start with a sketchbook and a pencil.
No, seriously. If you don't know what you ultimately want your end product to look like it doesn't matter what software you use. Once you have an idea of what works for you (sketchbook and pencil, markers and yellow bum wipe, whiteboard, etc.) and where the kinks in your workflow are you will be in a better position to chose some software.
Did you really like sketching out your ideas with broad-tipped markers? It may be that a more painterly-style (such as painter or Sketchbook Pro) will work for you. Did you spend a lot of time creating subtle gradients for your artwork? It may be Photoshop is a better bet. Did you pull the architect's square out and use it for a lot of perfectly straight lines? A vector-based piece of software such as Illustrator or even AutoCAD (since you said "3D like drawings").
Next, start with free versions of each type (GIMP, Inscape, etc.) and see if you like the workflow that's presented in the UI. It may be that you settle on one particular approach because you work better in a specific environment. Money paid for software is worth it in the long run only if you use that software.