i have another problem.. i want that a specific quest begins at the end of a dialogue, i did so well (correct me and help me step by step if I'm wrong, please):
I have open the topic info box > i have typed ";" in the papyrus fragment > i have pressed "properties" after selecting the new generated script > I have add a property > I have only set the type (quest) and i have pressed "OK" > i have edited the property's value, selecting the quest i wanted to start > I have saved > I have erased ";" in the fragment box and i have typed ".Start()" without < >.
So i have compiled the script, but the compilation is failed... how should i do?

Care to post the source of the offending script and the error message you get on the failure?

This is the error:

Starting 1 compile threads for 1 files...
Compiling "TIF__00023EE1"...
C:\Program Files\Bethesda Softworks\The Elder Scrolls V Skyrim\Data\Scripts\Source\temp\TIF__00023EE1.psc(18,15): script property NewProperty already defined
C:\Program Files\Bethesda Softworks\The Elder Scrolls V Skyrim\Data\Scripts\Source\temp\TIF__00023EE1.psc(18,15): script variable ::NewProperty_var already defined
C:\Program Files\Bethesda Softworks\The Elder Scrolls V Skyrim\Data\Scripts\Source\temp\TIF__00023EE1.psc(18,15): script property NewProperty already has a get function defined
C:\Program Files\Bethesda Softworks\The Elder Scrolls V Skyrim\Data\Scripts\Source\temp\TIF__00023EE1.psc(18,15): script property NewProperty already has a set function defined
C:\Program Files\Bethesda Softworks\The Elder Scrolls V Skyrim\Data\Scripts\Source\temp\TIF__00023EE1.psc(20,15): script property NewProperty already defined
C:\Program Files\Bethesda Softworks\The Elder Scrolls V Skyrim\Data\Scripts\Source\temp\TIF__00023EE1.psc(20,15): script variable ::NewProperty_var already defined
C:\Program Files\Bethesda Softworks\The Elder Scrolls V Skyrim\Data\Scripts\Source\temp\TIF__00023EE1.psc(20,15): script property NewProperty already has a get function defined
C:\Program Files\Bethesda Softworks\The Elder Scrolls V Skyrim\Data\Scripts\Source\temp\TIF__00023EE1.psc(20,15): script property NewProperty already has a set function defined
C:\Program Files\Bethesda Softworks\The Elder Scrolls V Skyrim\Data\Scripts\Source\temp\TIF__00023EE1.psc(22,15): script property NewProperty already defined
C:\Program Files\Bethesda Softworks\The Elder Scrolls V Skyrim\Data\Scripts\Source\temp\TIF__00023EE1.psc(22,15): script variable ::NewProperty_var already defined
C:\Program Files\Bethesda Softworks\The Elder Scrolls V Skyrim\Data\Scripts\Source\temp\TIF__00023EE1.psc(22,15): script property NewProperty already has a get function defined
C:\Program Files\Bethesda Softworks\The Elder Scrolls V Skyrim\Data\Scripts\Source\temp\TIF__00023EE1.psc(22,15): script property NewProperty already has a set function defined
C:\Program Files\Bethesda Softworks\The Elder Scrolls V Skyrim\Data\Scripts\Source\temp\TIF__00023EE1.psc(24,15): script property NewProperty already defined
C:\Program Files\Bethesda Softworks\The Elder Scrolls V Skyrim\Data\Scripts\Source\temp\TIF__00023EE1.psc(24,15): script variable ::NewProperty_var already defined
C:\Program Files\Bethesda Softworks\The Elder Scrolls V Skyrim\Data\Scripts\Source\temp\TIF__00023EE1.psc(24,15): script property NewProperty already has a get function defined
C:\Program Files\Bethesda Softworks\The Elder Scrolls V Skyrim\Data\Scripts\Source\temp\TIF__00023EE1.psc(24,15): script property NewProperty already has a set function defined
No output generated for TIF__00023EE1, compilation failed.

Batch compile of 1 files finished. 0 succeeded, 1 failed.
Failed on TIF__00023EE1

Looks like it's saying in your topic info script fragment "TIF__00023EE1.psc", you declared the property "NewProperty" 4 times.

Yeah, when defining a proprerty, you need to give it a name, and only define each one once.

ok...but how to script the quest?

We don't know...it all depends on what you want your quest to do. We lack the telepathic ability to discern what you are attempting to do, you know

the problem is that i have no idea where to start: i want to start a quest after the courier has finished to talk, without having to read the letter that gives you.

Try http://www.youtube.com/watch?v=Qfcet5hf5bs..pay special attention to the script he puts on the stools...you will want a similar script on the note the courier gives that starts the quest when the player receives it.