Change jsp on button click

You have several options, I'll start from the easiest:

1- Change the input buttons to links, you can style them with css so they look like buttons:

<a href="CreateCourse.jsp">Creazione Nuovo Corso</a>

instead of

<input type="button" value="Creazione Nuovo Corso" name="CreateCourse" />

2- Use javascript to change the action of the form depending on the button you click:

<input type="button" value="Creazione Nuovo Corso" name="CreateCourse" 
onclick="document.forms[0].action = 'CreateCourse.jsp'; return true;" />

3- Use a servlet or JSP to handle the request and redirect or forward to the appropriate JSP page.

If all you are looking for is navigation to page 2 and 3 from page one, replace the buttons with anchor elements as below:

<form name="TrainerMenu" action="TrainerMenu" method="get">

<h1>Benvenuto in LESSON! Scegli l'operazione da effettuare:</h1>
<a href="Page2.jsp" id="CreateCourse" >Creazione Nuovo Corso</a>&nbsp;
<a href="Page3.jsp" id="AuthorizationManager">Gestione Autorizzazioni</a>
<input type="button" value="" name="AuthorizationManager" />
</form>

If for some reason you need to use buttons, try this:

<form name="TrainerMenu" action="TrainerMenu" method="get">

   <h1>Benvenuto in LESSON! Scegli l'operazione da effettuare:</h1>
   <input type="button" value="Creazione Nuovo Corso" name="CreateCourse"
    onclick="openPage('Page2.jsp')"/>
   <input type="button" value="Gestione Autorizzazioni" name="AuthorizationManager"
    onclick="openPage('Page3.jsp')" />

</form>
<script type="text/javascript">
 function openPage(pageURL)
 {
 window.location.href = pageURL;
 }
</script>

The simplest way you can do this is to use java script. For example, <input type="button" value="load" onclick="window.location='userpage.jsp'" >

You could make those submit buttons and inside the servlet your are submitting the form to you could test the name of the button which was pressed and render the corresponding jsp page.

<input type="submit" value="Creazione Nuovo Corso" name="CreateCourse" />
<input type="submit" value="Gestione Autorizzazioni" name="AuthorizationManager" />

Inside the TrainerMenu servlet if request.getParameter("CreateCourse") is not empty then the first button was clicked and you could render the corresponding jsp.