Hi Luca,
Welcome to DNN!
You don't have very specific questions in your post, so I tell you how I started some years ago:
First I found DNN on a CD coming with a magazine. I was quite experienced in ASP and ASP.Net, but have not touched DotNetNuke before. I was excited about it, so I decided to learn how to use it. I took an existing skin (theme) and tried to change it (colors, measures etc.).
Next I took an existing module (which was written in VB) and rewrote it from scratch in C# - and by the time added some new functionality.
I asked the community (here in the forum) whenever I had some specific problems or did run into something I did not understand. People here are very helpful indeed.
I started to shared the things I knew or found out here in the forum and other places helping people to understand DNN, reported bugs on http://support.dnnsoftware.com, engaged myself in the community (eg. http://dnn-connect.org or the German speaking usergroup - try to find one in your country!), found a lot of new friends etc. and became DNN MVP in the last three years!
I wish you success with this fantastic system and
Happy DNNing!
Michael