Table: how to merge two cells?

Try \multicolumn{2}{|l|}{Text} \tabularnewline in the relevant row. \multicolumn{n}{q}{foo} merges n cells with type q and places foo as content.

Side note: Using booktabs and vertical lines in a table is not the best design. I would also use left alignment for the first column.

\documentclass[11pt,a4paper]{article}
\usepackage[T1]{fontenc}                
\usepackage[utf8]{inputenc}             
\usepackage[english,italian]{babel}
\usepackage{booktabs}
\usepackage{caption}
\usepackage[bindingoffset=1.5cm, left=3cm, right=3cm, top=3cm, bottom=3cm]{geometry}

\begin{document}
  \title{Title}
  \author{Me}
  %\date{10 dicembre 2015}

  \maketitle

  \begin{center}
    \begin{tabular}{|c|c|}
      \hline
      \multicolumn{2}{|l|}{Text} \tabularnewline
      \hline 
      Diametro fori (mm) & 4 \tabularnewline
      \hline 
      Passo x (mm) & 24\tabularnewline
      \hline
      Passo y (mm) & 24\tabularnewline
      \hline
      Altezza canale (mm) & 10\tabularnewline
      \hline
      Lunghezza canale (mm) & 146\tabularnewline
      \hline
      Numero fori & 39\tabularnewline
      \hline
    \end{tabular}
    \captionof{table}{Caratteristiche della piastra e del canale}
    \label{tab:dati_canale-piastra}
  \end{center}
\end{document}

Since you're already loading the booktabs package, you should give serious thought to improving the overall "look" of the table by (a) getting rid of all vertical lines, (b) getting rid of most horizontal lines, and (c) using the macros \toprule, \midrule, and \bottomrule of the booktabs package for the remaining three horizontal lines. With these changes in place, figuring out how to get rid of the unwanted central vertical divider in the header row becomes unnecessary. Note that, in the table below, I also suggest not using c (centering) for both columns; instead, I recommend l (left-alignment) for the first column and r (right-alignment) for the second column.

\documentclass[11pt,a4paper]{article}
\usepackage[T1]{fontenc}                
\usepackage[utf8]{inputenc}             
\usepackage[english,italian]{babel}
\usepackage{booktabs}
\usepackage{caption}
\usepackage[bindingoffset=1.5cm, margin=3cm]{geometry}

\begin{document}

\begin{center}
\begin{tabular}{@{} lr @{}}
\toprule
Text  \\
\midrule
Diametro fori (mm) & 4\\
Passo x (mm) & 24\\
Passo y (mm) & 24\\
Altezza canale (mm) & 10\\
Lunghezza canale (mm) & 146\\
Numero fori & 39\\
\bottomrule
\end{tabular}
\captionof{table}{Caratteristiche della piastra e del canale}
\label{tab:dati_canale-piastra}
\end{center}

\end{document}

You have to use multicolumn command:

\documentclass{book}
\begin{document}

\begin{table}
\begin{tabular}{|c|c|}
\multicolumn{2}{|c|}{\textbf{TITLE}} \\\hline
Value1 & Value2 \\\hline
Value1 & Value2 \\\hline
Value1 & Value2 \\\hline
\end{tabular}
\end{table}

\end{document}

UPDATE: As correctly mentioned below, you don't need the multirow package to use the multicolumn command.