I'm German. The process of submitting manuscripts is almost identical to the one in the USA. I assume it will be similar in Italy, too. What you are expected to do is usually:
Find an appropriate publisher. In your case that would be one that publishes SF, Fantasy, and/or fantastic literature, ideally in a similar style. (If you don't know the right publisher, then go to a bookstore and educate yourself.)
Look at their website. It is very likely that they give detailed instructions on how they expect manuscripts to be submitted to them and to whom.
Follow those instructions closely.
If you cannot find instructions, do this:
a. Write a polite and brief application, as if for a job. Explain why you chose that publisher. Describe your publication history (in your case, write that you haven't yet published anything).
b. Attach a two to five page synopsis of your novel. This synopsis must describe all the plot-relevant characters, tell everything that happens, and explain the resolution of your plot. That is, do not try to create suspense by keeping the end secret, as you would with readers. The publisher wants to know if you managed to create a satisfying whole, and for that they need to know all of your text.
c. Finally, include the first three chapters or 50 pages, whichever comes first.
d. Submit by email to the editor in charge of SF, F, or fantastic literature, if you can get their name (maybe call the main office and ask).
Wait eight weeks. Do not call. If you hear nothing, they are not interested.
Submitting to an agent works the same, except that you must tell them if you submit elsewhere. You need not include this information when submitting to a publisher directly.
If you know someone in publishing, try to have them recommend you.
Regarding some of the comments:
A note on finding agents and publishers
Ideally, you read what you write and you know the market. If you don't, go to bookstores, ideally more than one and, if possible, repeatedly over the course of several years, and learn which publishers have books (of your genre) in the bookstores. Those are the publishers you want to get published by.
If you publish in a niche that not every bookstore represents, find review publications relevant to that niche and see which publishers they review.
Along with your list of publishers, you will have aggregated a list of authors these publishers publish (in your genre). Now perform a websearch for the name of that author and "literary agency". Agents list their authors on their websites, so you should find every author's agent this way, unless that author does not have an agent.
A note on payments
German author Andreas Eschbach says on his website:
§1: Money always flows from agent to author – never in the opposite direction.
§2: There are no exceptions to rule 1.
§3: If you believe that your case must be an exception, you are mistaken.
I have heard and read similar advice from all kinds of people in the publishing industry, including publishers, editors, agents, and authors.
A note on professionalism
Publishers work and they want their job to run smoothly. They perceive your manuscript submission as a job application. So present yourself as reliable and capable. Take some care and time when you prepare your submission.
German agency AVN even call their submission guidelines "application as author".
A note on following up on your manuscript submission
Do not call the agency or publisher. This may be different in other countries, but German agencies and publishers explicitly tell you not to call them. For example, agency AVN says:
Please do not call us. If we are interested, we will contact you.
Publisher DuMont asks authors:
Please desist from inquiries regarding the state of our review of your manuscript.
Simply do not forget your contact information (see the note on professionalism above).