@UmbertoTosi Hey there.
Wow. You weren't kidding when you said you had some questions, heh. I'll see if I can tackle this.
1. About your copyright: Yes you own all the rights to your work, For more knowledge on this, the content guidelines touches briefly upon this subject. http://www.wattpad.com/guidelines - Basically, when you upload your work, you can chose what copyright it's under, whether it be all rights reserved, or common license. Wattpad do not 'own' your story, you're just giving them permission to broadcast it.
2. Writers may share what they wish to, as long as they are the copyright holders of the work, some published stories have been taken partially down, and only having a sample on here, others use Wattpad as a way of building a fan base by making the first book available for free. For more advice on how to go about Wattpad as a published writer you can check: http://www.wattpad.com/club/32-professional-writers which is a forum for published authors to talk about stuff.
3. Yes, you can link to your site and to pages where to buy your works (such as amazon) in your profile text, or on the stories itself. There's some different rules for the forums, but basically, keep the links to where they applies.
4. You decide for how long you may want to share your work, you can always take it down if you desire.
5. Wattpad broadcasts through it's site and the application. Any device with internet connection can go on the browser, but smartphones and tablets can also download the application, which enables offline-reading too, thus making your work reach a wider audience.
And finally, a guide is in the makings from the Wattpad team itself, it haven't been launched yet, but it will soon. If you have troubles with anything you may ask in the forums, or message me. However, the best advice is to just jump right in, participate in discussions in the clubs, and meet other writers.
I hope this helped, and glad to have you here. :)
- Maja