Create a document (glosario.tex) with all the definitions, for me most of them are acronyms so I have defined them the following way: \acro{CCTV}{\emph{Close Circuit Television}}
You seem to be mixing up packages. glossaries doesn't define \acro.
\usepackage[toc]{glossaries} % Load the package with the acronym option
This isn't loading the package with the acronym option. It's loading it with the toc option.
\makeglossaries{glosario} % Generate the glossary
The \makeglossaries command doesn't have an argument
Your glosario.tex file needs to be loaded using \loadglsentries. Example glossario.tex:
\newacronym{CCTV}{CCTV}{Closed Circuit Television}
Main document:
\documentclass{article}
\usepackage[acronym,toc]{glossaries}
\makeglossaries
% Define a style the emphasizes the long form:
\newacronymstyle{em-long-short}
{%
\GlsUseAcrEntryDispStyle{long-short}%
}%
{%
\GlsUseAcrStyleDefs{long-short}%
\renewcommand*{\genacrfullformat}[2]{%
\emph{\glsentrylong{##1}}##2\space
(\firstacronymfont{\glsentryshort{##1}})%
}%
\renewcommand*{\Genacrfullformat}[2]{%
\emph{\Glsentrylong{##1}}##2\space
(\firstacronymfont{\glsentryshort{##1}})%
}%
\renewcommand*{\genplacrfullformat}[2]{%
\emph{\glsentrylongpl{##1}}##2\space
(\firstacronymfont{\glsentryshortpl{##1}})%
}%
\renewcommand*{\Genplacrfullformat}[2]{%
\emph{\Glsentrylongpl{##1}}##2\space
(\firstacronymfont{\Glsentryshortpl{##1}})%
}%
}
\setacronymstyle{em-long-short}
\loadglsentries{glosario}
\begin{document}
First use: \gls{CCTV}. Next use: \gls{CCTV}.
\printglossary[type=\acronymtype]
\end{document}
Steps to build this document: latex, makeglossaries, latex (or pdflatex instead of latex). If you can't work out how to run makeglossaries, just add automake to the package option list:
\usepackage[automake,toc,acronym]{glossaries}
This produces:

Alternatively:
File glosario.tex:
\newabbreviation{CCTV}{CCTV}{Closed Circuit Television}
Main file:
\documentclass{article}
\usepackage[automake,abbreviations]{glossaries-extra}
\makeglossaries
\setabbreviationstyle{long-short}
\renewcommand{\glsfirstlongdefaultfont}[1]{\emph{#1}}
\loadglsentries{glosario}
\begin{document}
First use: \gls{CCTV}. Next use: \gls{CCTV}.
\printabbreviations
\end{document}
This produces:

glossariesdoesn't define\acro. Also,\makeglossariesdoesn't have an argument. You need to loadglosario.texwith\loadglsentries. Try the beginner's guide for some examples. – Nicola Talbot Apr 21 '16 at 09:20