<form method="GET" action='UserController?action=searchAlbum&searchItem' id="searchbox" >
Try this in place
<form method="GET" action='UserController' name="searchbox" >
You don't need to worry about the parameters, they will be appended automatically when the form is submitted.
And i do not understand what you are trying to do here as follows
<option value="searchArtist searchItem = <%= request.getParameter("searchItem") %>">Artist</option>
here also it seems you had forgotten to name your input field
<input ="searchItem" type="text" value="Type search items here">