I have defined fontsize, and even with your answered question help I solved the situation in this way:
\usepackage{titlesec}
\titleformat{\chapter}
{\normalfont\fontfamily{phv}\fontsize{16}{19}\bfseries}{\thechapter}{1em}{}
\titleformat{\section}
{\normalfont\fontfamily{phv}\fontsize{12}{17}\bfseries}{\thesection}{1em}{}
\titleformat{\subsection}
{\normalfont\fontfamily{phv}\fontsize{12}{17}\bfseries}{\thesubsection}{1em}{}
Now i have to add only to the subsection an italic style .How can I do?? Thank you so much for your attention and your precious help!
\documentclass...
,\begin{document}... \end{document}
etc. That said, just add\itshape
after\bfseries
to the format for\subsection
.